runtime: make a test more robust

The issue is discovered during testing of a change to runtime.
Even if it is unlikely to happen, the comment can safe an hour
next person who hits it.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/116790043

src/pkg/runtime/proc_test.go

@@ -101,6 +101,10 @@ func TestGoroutineParallelism(t *testing.T) {
 		N = 3
 	}
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(P))
+	// If runtime triggers a forced GC during this test then it will deadlock,
+	// since the goroutines can't be stopped/preempted.
+	// So give this test as much time as possible.
+	runtime.GC()
 	for try := 0; try < N; try++ {
 		done := make(chan bool)
 		x := uint32(0)