Pass Config to NewTestContext instead of ctx.Register

Prepare for using Config when adding singletons by passing
Config to NewTestContext and NewContext instead of to ctx.Register.
This will enable a followup change to store SingletonMakeVarsProviders
registered on the Context in the Config, which is necessary to run
multiple tests in parallel without data races.

Test: all soong tests
Change-Id: Id229629a4e42ff4487d317241673837726c075fc
diff --git a/android/prebuilt_test.go b/android/prebuilt_test.go
index 854395e..9ac3875 100644
--- a/android/prebuilt_test.go
+++ b/android/prebuilt_test.go
@@ -288,10 +288,10 @@
 				{Windows, Arch{ArchType: X86_64}, NativeBridgeDisabled, "", "", true},
 			}
 
-			ctx := NewTestArchContext()
+			ctx := NewTestArchContext(config)
 			registerTestPrebuiltBuildComponents(ctx)
 			ctx.RegisterModuleType("filegroup", FileGroupFactory)
-			ctx.Register(config)
+			ctx.Register()
 
 			_, errs := ctx.ParseBlueprintsFiles("Android.bp")
 			FailIfErrored(t, errs)