Add test for soong/testing/test_spec.

This CL adds test for test_spec.go and all_test_specs.go (singleton).

Bug: 296873595
Test: m nothing --no-skip-soong-tests -j96

Change-Id: I5010c68512e75d1b9a337c02da86faac15e376fe
diff --git a/java/Android.bp b/java/Android.bp
index 29c0957..cf96871 100644
--- a/java/Android.bp
+++ b/java/Android.bp
@@ -113,6 +113,7 @@
         "sdk_library_test.go",
         "system_modules_test.go",
         "systemserver_classpath_fragment_test.go",
+        "test_spec_test.go",
     ],
     pluginFor: ["soong_build"],
 }