os/exec: deflake TestStdinCloseRace

Stop reporting errors from cmd.Process.Kill; they don't matter for
purposes of this test, and they can occur if the process exits quickly.

Fixes #19211.
Fixes #19213.

Change-Id: I1a0bb9170220ca69199abb8e8811b1dde43e1897
Reviewed-on: https://go-review.googlesource.com/37309
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/exec/exec_test.go

@@ -263,9 +263,13 @@ func TestStdinCloseRace(t *testing.T) {
 		t.Fatalf("Start: %v", err)
 	}
 	go func() {
-		if err := cmd.Process.Kill(); err != nil {
-			t.Errorf("Kill: %v", err)
-		}
+		// We don't check the error return of Kill. It is
+		// possible that the process has already exited, in
+		// which case Kill will return an error "process
+		// already finished". The purpose of this test is to
+		// see whether the race detector reports an error; it
+		// doesn't matter whether this Kill succeeds or not.
+		cmd.Process.Kill()
 	}()
 	go func() {
 		// Send the wrong string, so that the child fails even