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/testing/all_test_specs.go b/testing/all_test_specs.go
index 2ecf15f..9d4645b 100644
--- a/testing/all_test_specs.go
+++ b/testing/all_test_specs.go
@@ -21,10 +21,10 @@
 	var intermediateMetadataPaths android.Paths
 
 	ctx.VisitAllModules(func(module android.Module) {
-		if !ctx.ModuleHasProvider(module, testSpecProviderKey) {
+		if !ctx.ModuleHasProvider(module, TestSpecProviderKey) {
 			return
 		}
-		intermediateMetadataPaths = append(intermediateMetadataPaths, ctx.ModuleProvider(module, testSpecProviderKey).(testSpecProviderData).IntermediatePath)
+		intermediateMetadataPaths = append(intermediateMetadataPaths, ctx.ModuleProvider(module, TestSpecProviderKey).(TestSpecProviderData).IntermediatePath)
 	})
 
 	rspFile := android.PathForOutput(ctx, fileContainingFilePaths)