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: I364677f09286652e4577bf9e0d16481998dfd258
diff --git a/fsgen/filesystem_creator_test.go b/fsgen/filesystem_creator_test.go
index 6bb0e77..554b66b 100644
--- a/fsgen/filesystem_creator_test.go
+++ b/fsgen/filesystem_creator_test.go
@@ -44,12 +44,13 @@
 		}),
 		android.FixtureMergeMockFs(android.MockFS{
 			"external/avb/test/data/testkey_rsa4096.pem": nil,
+			"build/soong/fsgen/Android.bp": []byte(`
+			soong_filesystem_creator {
+				name: "foo",
+			}
+			`),
 		}),
-	).RunTestWithBp(t, `
-	soong_filesystem_creator {
-		name: "foo",
-	}
-	`)
+	).RunTest(t)
 
 	fooSystem := result.ModuleForTests("test_product_generated_system_image", "android_common").Module().(interface {
 		FsProps() filesystem.FilesystemProperties