Add test for prebuilt_apis creation of system modules

Previously, there were no tests for this (outside uses in other tests).
This change adds a test that uses the fixture preparers that create a
prebuilt_api in order to make prebuilt APIs available. That ensures
that both the prebuilt_api is working as expected and the preparer
creates a realistic test environment.

Bug: 204189791
Test: m nothing
Change-Id: I57352aa00f7b268e5286be92f177764dd63ba7e8

java/Android.bp

@@ -93,6 +93,7 @@ bootstrap_go_package {
         "platform_bootclasspath_test.go",
         "platform_compat_config_test.go",
         "plugin_test.go",
+        "prebuilt_apis_test.go",
         "rro_test.go",
         "sdk_test.go",
         "sdk_library_test.go",

java/prebuilt_apis_test.go

@@ -0,0 +1,52 @@
+// Copyright 2021 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package java
+
+import (
+	"sort"
+	"strings"
+	"testing"
+
+	"android/soong/android"
+	"github.com/google/blueprint"
+)
+
+func TestPrebuiltApis_SystemModulesCreation(t *testing.T) {
+	result := android.GroupFixturePreparers(
+		prepareForJavaTest,
+		FixtureWithPrebuiltApis(map[string][]string{
+			"31":      {},
+			"current": {},
+		}),
+	).RunTest(t)
+
+	sdkSystemModules := []string{}
+	result.VisitAllModules(func(module blueprint.Module) {
+		name := android.RemoveOptionalPrebuiltPrefix(module.Name())
+		if strings.HasPrefix(name, "sdk_") && strings.HasSuffix(name, "_system_modules") {
+			sdkSystemModules = append(sdkSystemModules, name)
+		}
+	})
+	sort.Strings(sdkSystemModules)
+	expected := []string{
+		// 31 only has public system modules.
+		"sdk_public_31_system_modules",
+
+		// current only has public system modules.
+		"sdk_public_current_system_modules",
+	}
+	sort.Strings(expected)
+	android.AssertArrayString(t, "sdk system modules", expected, sdkSystemModules)
+}