mounttest: wait for Release after every Read to stop using in use files under Windows

cmd/mountlib/mounttest/fs.go

@@ -325,6 +325,7 @@ func (r *Run) readFile(t *testing.T, filepath string) string {
 	filepath = r.path(filepath)
 	result, err := ioutil.ReadFile(filepath)
 	require.NoError(t, err)
+	time.Sleep(100 * time.Millisecond) // FIXME wait for Release
 	return string(result)
 }