Create soong-generated filesystem diff test

Running `m soong_generated_filesystem_tests` will diff the
soong-generated system image with the make-generated one.

Currently, this test fails, but we can start working on narrowing it
down now.

Bug: 371351099
Test: m soong_generated_filesystem_tests
Change-Id: I6e341607415b3676e758417a587f69c247652c3a
diff --git a/fsgen/Android.bp b/fsgen/Android.bp
index aa8881f..9fa9557 100644
--- a/fsgen/Android.bp
+++ b/fsgen/Android.bp
@@ -19,3 +19,7 @@
     ],
     pluginFor: ["soong_build"],
 }
+
+soong_filesystem_creator {
+    name: "soong_filesystem_creator",
+}