[CHORE] Move test related function to own package

- Go's deadcode eliminator is quite simple, if you put a public function
in a package `aa/bb` that is used only by tests, it would still be built
if package `aa/bb` was imported. This means that if such functions use
libraries relevant only to tests that those libraries would still be
be built and increase the binary size of a Go binary.
- This is also the case with Forgejo, `models/migrations/base/tests.go`
contained functions exclusively used by tests which (skipping some steps
here) imports https://github.com/ClickHouse/clickhouse-go, which is
2MiB. The `code.gitea.io/gitea/models/migrations/base` package is
imported by `cmd/doctor` and thus the code of the clickhouse library is
also built and included in the Forgejo binary, although entirely unused
and not reachable.
- This patch moves the test-related functions to their own package, so
Go's deadcode eliminator knows not to build the test-related functions
and thus reduces the size of the Forgejo binary.
- It is not possible to move this to a `_test.go` file because Go does
not allow importing functions from such files, so any test helper
function must be in a non-test package and file.
- Reduction of size (built with `TAGS="sqlite sqlite_unlock_notify" make
build`):
  - Before: 95912040 bytes (92M)
  - After: 92306888 bytes (89M)

.deadcode-out

@@ -60,13 +60,6 @@ code.gitea.io/gitea/models/issues
 	IsErrIssueWasClosed
 	ChangeMilestoneStatus
 
-code.gitea.io/gitea/models/migrations/base
-	removeAllWithRetry
-	newXORMEngine
-	deleteDB
-	PrepareTestEnv
-	MainTest
-
 code.gitea.io/gitea/models/organization
 	GetTeamNamesByID
 	UpdateTeamUnits
@@ -93,36 +86,6 @@ code.gitea.io/gitea/models/repo
 	GetTopicByName
 	WatchRepoMode
 
-code.gitea.io/gitea/models/unittest
-	CheckConsistencyFor
-	checkForConsistency
-	GetXORMEngine
-	OverrideFixtures
-	InitFixtures
-	LoadFixtures
-	Copy
-	CopyDir
-	NewMockWebServer
-	NormalizedFullPath
-	FixturesDir
-	fatalTestError
-	InitSettings
-	MainTest
-	CreateTestEngine
-	PrepareTestDatabase
-	PrepareTestEnv
-	Cond
-	OrderBy
-	LoadBeanIfExists
-	BeanExists
-	AssertExistsAndLoadBean
-	GetCount
-	AssertNotExistsBean
-	AssertExistsIf
-	AssertSuccessfulInsert
-	AssertCount
-	AssertInt64InRange
-
 code.gitea.io/gitea/models/user
 	IsErrPrimaryEmailCannotDelete
 	ErrUserInactive.Error
@@ -273,22 +236,6 @@ code.gitea.io/gitea/modules/sync
 	StatusTable.Start
 	StatusTable.IsRunning
 
-code.gitea.io/gitea/modules/testlogger
-	testLoggerWriterCloser.pushT
-	testLoggerWriterCloser.Log
-	testLoggerWriterCloser.recordError
-	testLoggerWriterCloser.printMsg
-	testLoggerWriterCloser.popT
-	testLoggerWriterCloser.Reset
-	PrintCurrentTest
-	Printf
-	NewTestLoggerWriter
-	TestLogEventWriter.Base
-	TestLogEventWriter.GetLevel
-	TestLogEventWriter.GetWriterName
-	TestLogEventWriter.GetWriterType
-	TestLogEventWriter.Run
-
 code.gitea.io/gitea/modules/timeutil
 	GetExecutableModTime
 	MockSet