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/rust/testing.go b/rust/testing.go
index f2d4c5e..83b2828 100644
--- a/rust/testing.go
+++ b/rust/testing.go
@@ -77,6 +77,7 @@
 
 func CreateTestContext() *android.TestContext {
 	ctx := android.NewTestArchContext()
+	android.RegisterPrebuiltMutators(ctx)
 	cc.RegisterRequiredBuildComponentsForTest(ctx)
 	ctx.RegisterModuleType("genrule", genrule.GenRuleFactory)
 	ctx.RegisterModuleType("rust_binary", RustBinaryFactory)