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, 6226f464ce:

```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/test_utils.go

@@ -13,6 +13,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -312,9 +313,26 @@ func PrepareCleanPackageData(t testing.TB) {
 	require.NoError(t, storage.Clean(storage.Packages))
 }
 
+// inTestEnv keeps track if we are current inside a test environment, this is
+// used to detect if testing code tries to prepare a test environment more than
+// once.
+var inTestEnv atomic.Bool
+
 func PrepareTestEnv(t testing.TB, skip ...int) func() {
 	t.Helper()
-	deferFn := PrintCurrentTest(t, util.OptionalArg(skip)+1)
+
+	if !inTestEnv.CompareAndSwap(false, true) {
+		t.Fatal("Cannot prepare a test environment if you are already in a test environment. This is a bug in your testing code.")
+	}
+
+	deferPrintCurrentTest := PrintCurrentTest(t, util.OptionalArg(skip)+1)
+	deferFn := func() {
+		deferPrintCurrentTest()
+
+		if !inTestEnv.CompareAndSwap(true, false) {
+			t.Fatal("Tried to leave test environment, but we are no longer in a test environment. This should not happen.")
+		}
+	}
 
 	cancelProcesses(t, 30*time.Second)
 	t.Cleanup(func() { cancelProcesses(t, 0) }) // cancel remaining processes in a non-blocking way