Expose ProductVariables from TestConfig

In preparation for unexporting ProductVariables, explicitly return a
pointer to the structure from TestConfig / TestArchConfig.

Bug: 76168832
Test: m blueprint_tools
Change-Id: Iccfb4c912f8e0ee3f620cc1ee00f0cdc5cba7735
diff --git a/java/app_test.go b/java/app_test.go
index d3216bf..ba017a1 100644
--- a/java/app_test.go
+++ b/java/app_test.go
@@ -188,12 +188,12 @@
 	for _, testCase := range testEnforceRROTests {
 		t.Run(testCase.name, func(t *testing.T) {
 			config := testConfig(nil)
-			config.ProductVariables.ResourceOverlays = &resourceOverlays
+			config.TestProductVariables.ResourceOverlays = &resourceOverlays
 			if testCase.enforceRROTargets != nil {
-				config.ProductVariables.EnforceRROTargets = &testCase.enforceRROTargets
+				config.TestProductVariables.EnforceRROTargets = &testCase.enforceRROTargets
 			}
 			if testCase.enforceRROExcludedOverlays != nil {
-				config.ProductVariables.EnforceRROExcludedOverlays = &testCase.enforceRROExcludedOverlays
+				config.TestProductVariables.EnforceRROExcludedOverlays = &testCase.enforceRROExcludedOverlays
 			}
 
 			ctx := testAppContext(config, bp, fs)