Handle stopping goroutines of the MemoryLeakTestSuite.

We observed a race condition that sometimes goroutines are just closing but still measured by the goroutine Lookup of the following test.
We try to avoid this by adding `runtime.Gosched()` at the start of each test.

tests/util.go

@@ -39,6 +39,8 @@ type MemoryLeakTestSuite struct {
 }
 
 func (s *MemoryLeakTestSuite) SetupTest() {
+	// Without this first line we observed some goroutines just closing.
+	runtime.Gosched()
 	s.ExpectedGoroutingIncrease = 0
 	s.goroutinesBefore = &bytes.Buffer{}