gopls/internal/filewatcher: skip test for unsupported OS

For golang/go#74543

Change-Id: Ie0a58f39bc7ec53e2ee5d61818bb99c84181553e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/686822
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: h9jiang

gopls/internal/filewatcher/filewatcher_test.go

@@ -18,6 +18,12 @@ import (
 )
 
 func TestFileWatcher(t *testing.T) {
+	switch runtime.GOOS {
+	case "darwin", "linux", "windows":
+	default:
+		t.Skip("unsupported OS")
+	}
+
 	testCases := []struct {
 		name           string
 		goos           []string // if not empty, only run in these OS.