Add filegroup_test.go to sources

I don't think it was running before.

Test: yes
Change-Id: Ia3bf53ddcc1ddefd7a236122256594ba764ee8e5
diff --git a/android/Android.bp b/android/Android.bp
index 29a88f2..641c438 100644
--- a/android/Android.bp
+++ b/android/Android.bp
@@ -111,6 +111,7 @@
         "depset_test.go",
         "deptag_test.go",
         "expand_test.go",
+        "filegroup_test.go",
         "fixture_test.go",
         "gen_notice_test.go",
         "license_kind_test.go",