os: invoke processKiller synchronously in testKillProcess

Previously, testKillProcess needlessly invoked processKiller in a
separate goroutine and failed to wait for that goroutine to complete,
causing the calls to t.Fatalf in that goroutine to potentially occur
after the test function had already returned.

Fixes #43722

Change-Id: I5d03cb24af51bb73f0ff96419dac57ec39776967
Reviewed-on: https://go-review.googlesource.com/c/go/+/284153
Trust: Bryan C. Mills <[email protected]>
Trust: Jason A. Donenfeld <[email protected]>
Reviewed-by: Jason A. Donenfeld <[email protected]>
Run-TryBot: Jason A. Donenfeld <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: Bryan C. Mills

src/os/os_test.go

@@ -2298,21 +2298,23 @@ func TestLongPath(t *testing.T) {
 
 func testKillProcess(t *testing.T, processKiller func(p *Process)) {
 	testenv.MustHaveExec(t)
+	t.Parallel()
 
 	// Re-exec the test binary itself to emulate "sleep 1".
 	cmd := osexec.Command(Args[0], "-test.run", "TestSleep")
 	err := cmd.Start()
 	if err != nil {
 		t.Fatalf("Failed to start test process: %v", err)
 	}
-	go func() {
-		time.Sleep(100 * time.Millisecond)
-		processKiller(cmd.Process)
+
+	defer func() {
+		if err := cmd.Wait(); err == nil {
+			t.Errorf("Test process succeeded, but expected to fail")
+		}
 	}()
-	err = cmd.Wait()
-	if err == nil {
-		t.Errorf("Test process succeeded, but expected to fail")
-	}
+
+	time.Sleep(100 * time.Millisecond)
+	processKiller(cmd.Process)
 }
 
 // TestSleep emulates "sleep 1". It is a helper for testKillProcess, so we