Add integration testing infrastructure

Fix mutator registration for tests to allow different tests
in the same package to register different mutators.

Allow tests to track the resulting ModuleBuildParams objects
to use in assertions, and provide helpers for getting them.
For example:
    config := android.TestConfig(buildDir)
    ctx := android.NewTestContext()
    ctx.RegisterModuleType(...)
    ctx.MockFileSystem(...)
    ctx.ParseBlueprintsFile("Android.bp")
    ctx.PrepareBuildActions(config)
    ctx.Register()
    // Get the Inputs value passed to the javac rule for the foo module
    inputs := ctx.ModuleForTests("foo".Rule("javac").Inputs

Test: java_test.go
Change-Id: I10c82967f5f3586d2c176f169906b571ed82fc73

android/defaults.go

@@ -113,6 +113,11 @@ func (defaultable *DefaultableModule) applyDefaults(ctx TopDownMutatorContext,
 	}
 }
 
+func registerDefaultsPreArchMutators(ctx RegisterMutatorsContext) {
+	ctx.BottomUp("defaults_deps", defaultsDepsMutator).Parallel()
+	ctx.TopDown("defaults", defaultsMutator).Parallel()
+}
+
 func defaultsDepsMutator(ctx BottomUpMutatorContext) {
 	if defaultable, ok := ctx.Module().(Defaultable); ok {
 		ctx.AddDependency(ctx.Module(), DefaultsDepTag, defaultable.defaults().Defaults...)