sync: fix goroutine leak for when TestMutexFairness times out

If the timeout triggers before writing to the done channel, the
goroutine will be blocked waiting for a corresponding read that’s
no longer existent, thus a goroutine leak. This change fixes that by
using a buffered channel instead.

Change-Id: I9cf4067a58bc5a729ab31e4426edd78bd359e8e0
GitHub-Last-Rev: a7d811a7be6d875175a894e53d474aa0034e7d2c
GitHub-Pull-Request: golang/go#40236
Reviewed-on: https://go-review.googlesource.com/c/go/+/242902
Reviewed-by: Emmanuel Odeke <[email protected]>
Run-TryBot: Emmanuel Odeke <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

src/sync/mutex_test.go

@@ -194,7 +194,7 @@ func TestMutexFairness(t *testing.T) {
  }
  }
  }()
- done := make(chan bool)
+ done := make(chan bool, 1)
  go func() {
  for i := 0; i < 10; i++ {
  time.Sleep(100 * time.Microsecond)