Add filesystem_test.go

It has only one test function as a starting point.

Bug: n/a
Test: m (soong tests)
Change-Id: I785b096805014a40dfd600f7baaf884f4016c23c

filesystem/testing.go

@@ -0,0 +1,19 @@
+// Copyright 2021 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package filesystem
+
+import "android/soong/android"
+
+var PrepareForTestWithFilesystemBuildComponents = android.FixtureRegisterWithContext(registerBuildComponents)