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/markup/camo_test.go

@@ -7,12 +7,14 @@ import (
 	"testing"
 
 	"gitea.dev/modules/setting"
+	testModule "gitea.dev/modules/test"
 
 	"github.com/stretchr/testify/assert"
 )
 
 func TestCamoHandleLink(t *testing.T) {
-	setting.AppURL = "https://gitea.com"
+	defer testModule.MockVariableValue(&setting.AppURL, "https://gitea.com")()
+	defer testModule.MockVariableValue(&setting.Camo)()
 	// Test media proxy
 	setting.Camo.Enabled = true
 	setting.Camo.Always = false
@@ -42,7 +44,4 @@ func TestCamoHandleLink(t *testing.T) {
 	assert.Equal(t,
 		"https://image.proxy/eivin43gJwGVIjR9MiYYtFIk0mw/aHR0cDovL3Rlc3RpbWFnZXMub3JnL2ltZy5qcGc",
 		camoHandleLink("http://testimages.org/img.jpg"))
-
-	// Restore previous settings
-	setting.Camo.Enabled = false
 }

modules/markup/html_test.go

@@ -5,6 +5,7 @@ package markup_test
 
 import (
 	"io"
+	"maps"
 	"strings"
 	"testing"
 
@@ -326,8 +327,9 @@ func TestRender_email(t *testing.T) {
 }
 
 func TestRender_emoji(t *testing.T) {
-	setting.AppURL = markup.TestAppURL
-	setting.StaticURLPrefix = strings.TrimSuffix(markup.TestAppURL, "/")
+	defer testModule.MockVariableValue(&setting.AppURL, markup.TestAppURL)()
+	defer testModule.MockVariableValue(&setting.StaticURLPrefix, strings.TrimSuffix(markup.TestAppURL, "/"))()
+	defer testModule.MockVariableValue(&setting.UI.CustomEmojisMap, maps.Clone(setting.UI.CustomEmojisMap))()
 
 	test := func(input, expected string) {
 		expected = strings.ReplaceAll(expected, "&", "&")