testenv: kill subprocess if SIGQUIT doesn't do it

This makes testenv.RunWithTimeout first attempt to SIGQUIT the
subprocess to get a useful Go traceback, but if that doesn't work, it
sends a SIGKILL instead to make sure we tear down the subprocess. This
is potentially important for non-Go subprocesses.

For #37405.

Change-Id: I9e7e118dc5769ec3f45288a71658733bff30c9cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/370702
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/testenv/testenv.go

@@ -346,6 +346,13 @@ func RunWithTimeout(t testing.TB, cmd *exec.Cmd) ([]byte, error) {
 		case <-done:
 		case <-time.After(time.Duration(scale) * time.Minute):
 			p.Signal(Sigquit)
+			// If SIGQUIT doesn't do it after a little
+			// while, kill the process.
+			select {
+			case <-done:
+			case <-time.After(time.Duration(scale) * 30 * time.Second):
+				p.Signal(os.Kill)
+			}
 		}
 	}()