os/exec: avoid leaking a process in TestDoubleStartLeavesPipesOpen

Updates #52580.
For #50436.

Change-Id: I0929055ffca1ca429f6ebec7d877f4268bd1fda2
Reviewed-on: https://go-review.googlesource.com/c/go/+/436656
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Benny Siegert <[email protected]>

Author: Bryan C. Mills

src/os/exec/exec_test.go

@@ -1108,9 +1108,16 @@ func TestDoubleStartLeavesPipesOpen(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+
 	if err := cmd.Start(); err != nil {
 		t.Fatal(err)
 	}
+	t.Cleanup(func() {
+		if err := cmd.Wait(); err != nil {
+			t.Error(err)
+		}
+	})
+
 	if err := cmd.Start(); err == nil || !strings.HasSuffix(err.Error(), "already started") {
 		t.Fatalf("second call to Start returned a nil; want an 'already started' error")
 	}