Move functionality to test install rules into testing.go.

In preparation for test in the cc package.

Test: m nothing
Bug: 211770050
Change-Id: I3f6190e102c607a0b6246d78d7bde7fcffa21650
diff --git a/android/singleton_module_test.go b/android/singleton_module_test.go
index eb5554c..9d98478 100644
--- a/android/singleton_module_test.go
+++ b/android/singleton_module_test.go
@@ -46,8 +46,8 @@
 	PrepareForTestWithAndroidMk,
 	FixtureRegisterWithContext(func(ctx RegistrationContext) {
 		ctx.RegisterSingletonModuleType("test_singleton_module", testSingletonModuleFactory)
-		ctx.RegisterSingletonType("makevars", makeVarsSingletonFunc)
 	}),
+	PrepareForTestWithMakevars,
 )
 
 func TestSingletonModule(t *testing.T) {