fix(testutil): ensure FakeSink does not swallow logs (#25185)

`FakeSink` was silently capturing log entries without forwarding them to
`testing.TB.Log`. This made debugging test failures harder because logs
were invisible in `go test -v` output.

Store `testing.TB` in `FakeSink` and call `t.Log` on each entry, guarded
by a check to avoid logging after the test has finished.

Split out from #25012.

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testutil/fake_sink.go

@@ -12,21 +12,33 @@ import (
 // tests can assert on what was logged. It requires a testing.TB
 // as it is only meant for use in tests.
 type FakeSink struct {
+	t       testing.TB
 	mu      sync.RWMutex
 	entries []slog.SinkEntry
+	tDone   bool
 }
 
 // NewFakeSink returns a FakeSink ready for use.
-func NewFakeSink(_ testing.TB) *FakeSink {
-	return &FakeSink{}
+func NewFakeSink(t testing.TB) *FakeSink {
+	fs := &FakeSink{t: t}
+	t.Cleanup(func() {
+		fs.mu.Lock()
+		fs.tDone = true
+		fs.mu.Unlock()
+	})
+	return fs
 }
 
 // LogEntry implements slog.Sink. It appends the entry to the
 // internal slice.
 func (s *FakeSink) LogEntry(_ context.Context, e slog.SinkEntry) {
 	s.mu.Lock()
-	defer s.mu.Unlock()
 	s.entries = append(s.entries, e)
+	shouldLog := !s.tDone
+	s.mu.Unlock()
+	if shouldLog {
+		s.t.Log(e.Message, e.Fields)
+	}
 }
 
 // Sync implements slog.Sink.