Add filesystem_test.go

It has only one test function as a starting point.

Bug: n/a
Test: m (soong tests)
Change-Id: I785b096805014a40dfd600f7baaf884f4016c23c
diff --git a/filesystem/filesystem.go b/filesystem/filesystem.go
index b2bd6bd..cf98717 100644
--- a/filesystem/filesystem.go
+++ b/filesystem/filesystem.go
@@ -26,7 +26,11 @@
 )
 
 func init() {
-	android.RegisterModuleType("android_filesystem", filesystemFactory)
+	registerBuildComponents(android.InitRegistrationContext)
+}
+
+func registerBuildComponents(ctx android.RegistrationContext) {
+	ctx.RegisterModuleType("android_filesystem", filesystemFactory)
 }
 
 type filesystem struct {