Move NDK test modules into cc/testing.go

Move the NDK modules defined in app_test.go into cc/testing.go in
preparation for using sdk_version: "current" in more tests.

Test: all soong tests
Change-Id: I80bc31225fb4562fce42781219cefbbd6affae06
diff --git a/sdk/testing.go b/sdk/testing.go
index 6102441..ae0620d 100644
--- a/sdk/testing.go
+++ b/sdk/testing.go
@@ -53,6 +53,8 @@
 		"myapex.pk8":                                 nil,
 	}
 
+	cc.GatherRequiredFilesForTest(mockFS)
+
 	for k, v := range fs {
 		mockFS[k] = v
 	}