fs: add TestRoMount

Also add a test if "ro" works directly in the fs
package.

Currently fails like this (will be fixed later):

	--- FAIL: TestRoMount (0.01s)
	    simple_test.go:116: read-only file syste

Change-Id: I8194360b1d092268784b9c2b17de274b86633aa2

fs/loopback_linux_test.go

@@ -380,3 +380,8 @@ func TestParallelDiropsHang(t *testing.T) {
 	wg.Wait()
 	server.Unmount()
 }
+
+func TestRoMount(t *testing.T) {
+	tc := newTestCase(t, &testOptions{ro: true})
+	defer tc.Clean()
+}

fs/simple_test.go

@@ -53,6 +53,7 @@ type testOptions struct {
 	attrCache     bool
 	suppressDebug bool
 	testDir       string
+	ro            bool
 }
 
 // newTestCase creates the directories `orig` and `mnt` inside a temporary
@@ -102,6 +103,9 @@ func newTestCase(t *testing.T, opts *testOptions) *testCase {
 	if !opts.suppressDebug {
 		mOpts.Debug = testutil.VerboseTest()
 	}
+	if opts.ro {
+		mOpts.Options = append(mOpts.Options, "ro")
+	}
 	tc.server, err = fuse.NewServer(tc.rawFS, tc.mntDir, mOpts)
 	if err != nil {
 		t.Fatal(err)