cmd/gogio: support running e2e with and without -race

Right now, we can only run the e2e gio app with -race, not without
-race, because the flag is hard-coded in the tests.

The reason for this change was that 'GOFLAGS=-race go test' would fail
with the JS test, since js/wasm doesn't support the race detector. Fix
that by skipping the JS test when -race is used.

Now, we can run multiple levels of -race:

	go test               # no -race at all
	go test -race         # -race for the tests, not the e2e gio app
	GOFLAGS=-race go test # -race for everything (best-effort)

To detect the race detector being on, we use a file with a build tag.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

cmd/gogio/x11_test.go

@@ -69,7 +69,7 @@ func (d *X11TestDriver) Start(t_ *testing.T, path string, width, height int) (cl
 	cleanups = append(cleanups, func() { os.RemoveAll(dir) })
 
 	bin := filepath.Join(dir, "red")
-	cmd := exec.Command("go", "build", "-race", "-tags", "nowayland", "-o="+bin, path)
+	cmd := exec.Command("go", "build", "-tags", "nowayland", "-o="+bin, path)
 	if out, err := cmd.CombinedOutput(); err != nil {
 		d.t.Fatalf("could not build app: %s:\n%s", err, out)
 	}