Fix prebuilt mutator ordering in tests

Previously, the prebuilt mutators were added by the
cc.RegisterRequiredBuildComponentsForTest() function as a convenience
but unfortunately it lead to some of the mutators being in a different
order in the tests than in the normal build.

This change:
* Extracts the RegisterPrebuiltMutators() call from
  cc.RegisterRequiredBuildComponentsForTest()
* Makes sure that the prebuilt mutators are registered before the
  visibility gatherer and enforcer mutators.

Bug: 162505935
Test: m nothing
Change-Id: I7d959b558200b502f0a5e4653c41ea01414e142a
diff --git a/sdk/testing.go b/sdk/testing.go
index 34ea8f0..b53558d 100644
--- a/sdk/testing.go
+++ b/sdk/testing.go
@@ -97,6 +97,11 @@
 	ctx.PreArchMutators(android.RegisterVisibilityRuleChecker)
 	ctx.PreArchMutators(android.RegisterDefaultsPreArchMutators)
 	ctx.PreArchMutators(android.RegisterComponentsMutator)
+
+	android.RegisterPrebuiltMutators(ctx)
+
+	// Register these after the prebuilt mutators have been registered to match what
+	// happens at runtime.
 	ctx.PreArchMutators(android.RegisterVisibilityRuleGatherer)
 	ctx.PostDepsMutators(android.RegisterVisibilityRuleEnforcer)