test: fix unit tests failing on repeated runs (#39915)

`go test -count=2` failed in 12 packages because tests leaked state into
later runs.

- Reset SQLite sequences after each fixture load, like PostgreSQL
already does
- Prepare the database in tests that relied on earlier tests doing it
- Restore mutated globals with `test.MockVariableValue`

modules/setting/lfs_test.go

@@ -6,6 +6,8 @@ package setting
 import (
 	"testing"
 
+	"gitea.dev/modules/test"
+
 	"github.com/stretchr/testify/assert"
 )
 
@@ -103,6 +105,9 @@ STORAGE_TYPE = minio
 }
 
 func Test_LFSClientServerConfigs(t *testing.T) {
+	defer test.MockVariableValue(&LFS)()
+	defer test.MockVariableValue(&LFSClient)()
+
 	iniStr := `
 [server]
 LFS_MAX_BATCH_SIZE = 100

modules/setting/markup_test.go

@@ -6,10 +6,17 @@ package setting
 import (
 	"testing"
 
+	"gitea.dev/modules/test"
+
 	"github.com/stretchr/testify/assert"
 )
 
 func TestLoadMarkup(t *testing.T) {
+	defer test.MockVariableValue(&Markdown)()
+	defer test.MockVariableValue(&ExternalMarkupRenderers)()
+	defer test.MockVariableValue(&ExternalSanitizerRules)()
+	defer test.MockVariableValue(&MermaidMaxSourceCharacters)()
+
 	cfg, _ := NewConfigProviderFromData(``)
 	loadMarkupFrom(cfg)
 	assert.Equal(t, MarkdownMathCodeBlockOptions{ParseInlineDollar: true, ParseBlockDollar: true}, Markdown.MathCodeBlockOptions)

modules/setting/oauth2_test.go

@@ -60,6 +60,8 @@ func TestGetGeneralSigningSecretSave(t *testing.T) {
 }
 
 func TestOauth2DefaultApplications(t *testing.T) {
+	defer test.MockVariableValue(&OAuth2)()
+
 	cfg, _ := NewConfigProviderFromData(``)
 	loadOAuth2From(cfg)
 	assert.Equal(t, []string{"git-credential-oauth", "git-credential-manager", "tea", "gitea-app", "gitea-vscode"}, OAuth2.DefaultApplications)

modules/setting/security_test.go

@@ -6,10 +6,14 @@ package setting
 import (
 	"testing"
 
+	"gitea.dev/modules/test"
+
 	"github.com/stretchr/testify/assert"
 )
 
 func TestLoadSecurityFrom(t *testing.T) {
+	defer test.MockVariableValue(&Security)()
+
 	assert.Equal(t, "SAMEORIGIN", Security.XFrameOptions)
 	assert.Equal(t, "nosniff", Security.XContentTypeOptions)
 	assert.Equal(t, "lax", Security.EgressMode)