time: TestAfterStop flaky on solaris-amd64-oraclerel

```
--- FAIL: TestAfterStop (0.47s)
    sleep_test.go:244: failed to stop event 0
FAIL
FAIL	time	5.886s
```

[2019-11-12T05:35:33-99957b6/solaris-amd64-oraclerel](https://build.golang.org/log/6f535922ec3017d949228f14c1912eed3dd399fb)
[2019-10-11T18:07:53-24f9238/solaris-amd64-oraclerel](https://build.golang.org/log/75a2ed8f2e1e18feca78ba0d52376d3e259e7f5f)

CC @ianlancetaylor @rorth