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/test_spec.go b/testing/test_spec.go
index 1ad2768..c370f71 100644
--- a/testing/test_spec.go
+++ b/testing/test_spec.go
@@ -78,11 +78,11 @@
 }
 
 // Provider published by TestSpec
-type testSpecProviderData struct {
+type TestSpecProviderData struct {
 	IntermediatePath android.WritablePath
 }
 
-var testSpecProviderKey = blueprint.NewProvider(testSpecProviderData{})
+var TestSpecProviderKey = blueprint.NewProvider(TestSpecProviderData{})
 
 type TestModuleProviderData struct {
 }
@@ -120,7 +120,7 @@
 	android.WriteFileRule(ctx, intermediatePath, string(protoData))
 
 	ctx.SetProvider(
-		testSpecProviderKey, testSpecProviderData{
+		TestSpecProviderKey, TestSpecProviderData{
 			IntermediatePath: intermediatePath,
 		},
 	)