docs: add development setup guide (#37960)

Moves the "Hacking on Gitea" page out of the documentation website and into the repository as `docs/development.md`, so contributors find build and test instructions next to the code. The content has been cleaned up and corrected for in-repo use.

---------

Signed-off-by: bircni <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

docs/testing.md

@@ -0,0 +1,151 @@
+# Testing
+
+Gitea has four kinds of automated tests: backend unit tests, integration tests,
+end-to-end (e2e) tests, and migration tests. Local runs default to SQLite, so no
+extra services are required to get started.
+
+For prerequisites see [build-setup.md](build-setup.md); for the build workflow see
+[development.md](development.md).
+
+## Unit tests
+
+Backend unit tests live in `*_test.go` files next to the code they cover. Set
+`GITEA_TEST_LOG_SQL=1` to log all SQL statements executed during the tests.
+
+```bash
+make test-backend
+```
+
+To run a single backend test, use `go test` directly or the `#` selector:
+
+```bash
+go test -run '^TestName$' ./modulepath/
+make test-backend#TestName
+```
+
+Frontend unit tests run with [Vitest](https://vitest.dev/):
+
+```bash
+make test-frontend
+# single file:
+pnpm exec vitest <path-filter>
+```
+
+## Integration tests
+
+Integration tests exercise Gitea against a real database. They live in
+`tests/integration/` and require [Git LFS](https://git-lfs.com/) to be installed.
+The database is selected with `GITEA_TEST_DATABASE`; an empty value defaults to
+SQLite, which needs no external service:
+
+```bash
+make test-integration
+```
+
+Run a single integration test with the `#` selector:
+
+```bash
+make test-integration#TestName
+```
+
+If you hit errors such as a mismatched database version or SSH push failures, try a
+clean rebuild first:
+
+```bash
+make clean build
+```
+
+### Running against other databases
+
+Set `GITEA_TEST_DATABASE` together with the matching `TEST_*` connection variables.
+The commands below start a throwaway database container (press `Ctrl-C` to stop and
+remove it) and then run the tests against it.
+
+#### MySQL
+
+```bash
+docker run -e "MYSQL_DATABASE=test" -e "MYSQL_ALLOW_EMPTY_PASSWORD=yes" -p 3306:3306 --rm --name mysql mysql:latest
+```
+
+```bash
+GITEA_TEST_DATABASE=mysql TEST_MYSQL_HOST=localhost:3306 TEST_MYSQL_DBNAME=test TEST_MYSQL_USERNAME=root TEST_MYSQL_PASSWORD='' make test-integration
+```
+
+#### PostgreSQL
+
+PostgreSQL tests also use a MinIO container for object storage:
+
+```bash
+docker run -e "POSTGRES_DB=test" -e "POSTGRES_USER=postgres" -e "POSTGRES_PASSWORD=postgres" -p 5432:5432 --rm --name pgsql postgres:latest
+docker run --rm -p 9000:9000 -e MINIO_ROOT_USER=123456 -e MINIO_ROOT_PASSWORD=12345678 --name minio bitnamilegacy/minio:2023.8.31
+```
+
+```bash
+GITEA_TEST_DATABASE=pgsql TEST_MINIO_ENDPOINT=localhost:9000 TEST_PGSQL_HOST=localhost:5432 TEST_PGSQL_DBNAME=postgres TEST_PGSQL_USERNAME=postgres TEST_PGSQL_PASSWORD=postgres make test-integration
+```
+
+#### MSSQL
+
+```bash
+docker run -e "ACCEPT_EULA=Y" -e "MSSQL_PID=Standard" -e "SA_PASSWORD=MwantsaSecurePassword1" -p 1433:1433 --rm --name mssql microsoft/mssql-server-linux:latest
+```
+
+```bash
+GITEA_TEST_DATABASE=mssql TEST_MSSQL_HOST=localhost:1433 TEST_MSSQL_DBNAME=gitea_test TEST_MSSQL_USERNAME=sa TEST_MSSQL_PASSWORD=MwantsaSecurePassword1 make test-integration
+```
+
+### Running the database test workflow with Gitea Runner
+
+The CI database test jobs can be run locally with
+[Gitea Runner](https://gitea.com/gitea/runner). Running every job is
+resource-intensive and not recommended:
+
+```bash
+gitea-runner exec -W ./.github/workflows/pull-db-tests.yml --event=pull_request --default-actions-url="https://github.com" -i catthehacker/ubuntu:runner-latest
+```
+
+List the available job names, then run a single one:
+
+```bash
+gitea-runner exec -W ./.github/workflows/pull-db-tests.yml --event=pull_request --default-actions-url="https://github.com" -i catthehacker/ubuntu:runner-latest -l
+gitea-runner exec -W ./.github/workflows/pull-db-tests.yml --event=pull_request --default-actions-url="https://github.com" -i catthehacker/ubuntu:runner-latest -j <job_name>
+```
+
+## End-to-end tests
+
+End-to-end tests drive a running Gitea instance with [Playwright](https://playwright.dev/):
+
+```bash
+make test-e2e
+```
+
+To run a single e2e test file, pass it via `GITEA_TEST_E2E_FLAGS`:
+
+```bash
+GITEA_TEST_E2E_FLAGS='<filepath>' make test-e2e
+```
+
+Useful environment variables:
+
+| Variable | Description |
+| :--- | :--- |
+| `GITEA_TEST_E2E_DEBUG` | When set, show the Gitea server output. |
+| `GITEA_TEST_E2E_FLAGS` | Additional flags passed to Playwright, e.g. `--ui`. |
+| `GITEA_TEST_E2E_TIMEOUT_FACTOR` | Timeout multiplier (default: 4 on CI, 1 locally). |
+
+## Migration tests
+
+If you change a database-persisted struct under `models/` you will usually need a
+new migration in `models/migrations/`. Run the migration tests with:
+
+```bash
+make test-migration
+```
+
+## Continuous integration
+
+CI runs the unit tests, runs the integration tests against every supported database,
+and tests migration from several recent Gitea versions. Please submit your pull
+request with additional unit and integration tests as appropriate. Prefer unit tests
+when the logic can be tested in isolation, and keep local integration and e2e tests
+fast (aim for sub-2s runtime).