feat: detect incorrect integration test functions (#8352)

- I have seen multiple times where a test function tries to prepare the
testing environment more than once, this can lead to bugs and false
positives of testing code. I would attribute this to lack of
documentation on how to write integration tests.
- To detect such cases, keep track when we are in a prepared test
environment and fail when some testing code is tries to once again
prepare the test environment.
- The message is logged to the function call that is requesting to
prepare the test environment, for example: `change_default_branch_test.go:19: Cannot prepare a test environment if you are already in a test environment. This is a bug in your testing code.`

A example of what this will be able to catch, 6226f464cec870991302c62a514d11ddb2066b69:

```go
func TestFoo(t *testing.T) {
	defer PrepareTestEnv(t)()

	t.Run("Bar", func(t *testing.T) {
		defer PrepareTestEnv(t)() // Should very likely be PrintCurrentTest
	})
}
```

```go
func TestBar(t *testing.T) {
	onGiteaRun(t, func(t *testing.T, _ *url.URL) {
		defer PrepareTestEnv(t)() // Already called by onGiteaRun.
	})
}
```

```go
func TestFooBar(t *testing.T) {
	defer PrepareTestEnv(t)() // This will be called by onGiteaRun later on and very unlikely to do this before the call to onGiteaRun.
	onGiteaRun(t, func(t *testing.T, _ *url.URL) {
		// [...]
	})
}
```

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8352
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

tests/integration/mirror_push_test.go

@@ -43,7 +43,6 @@ func TestMirrorPush(t *testing.T) {
 }
 
 func testMirrorPush(t *testing.T, u *url.URL) {
-	defer tests.PrepareTestEnv(t)()
 	defer test.MockVariableValue(&setting.Migrations.AllowLocalNetworks, true)()
 
 	require.NoError(t, migrations.Init())