Add test infrastructure for running the arch mutator

Add TestArchConfig and NewTestArchContext to create a test context
that will run the arch mutator, which is necessary to test any code
that is inside a ctx.Device() block or similar.

Test: next patch
Change-Id: Ieb1a0326bc27fc18ba88b8d37655a7e0c6870d6c

android/testing.go

@@ -27,6 +27,12 @@ func NewTestContext() *TestContext {
 	}
 }
 
+func NewTestArchContext() *TestContext {
+	ctx := NewTestContext()
+	ctx.preDeps = append(ctx.preDeps, registerArchMutator)
+	return ctx
+}
+
 type TestContext struct {
 	*blueprint.Context
 	preArch, preDeps, postDeps []RegisterMutatorFunc