Let tests override environment

Pass an environment to TestConfig that will be used for
all Config.Getenv calls.

Test: none
Change-Id: I683cd9c9e0db61c9bfd2adb27fca78f558f225c4

java/java_test.go

@@ -52,7 +52,7 @@ func TestMain(m *testing.M) {
 }
 
 func testJava(t *testing.T, bp string) *android.TestContext {
-	config := android.TestArchConfig(buildDir)
+	config := android.TestArchConfig(buildDir, nil)
 
 	ctx := android.NewTestArchContext()
 	ctx.RegisterModuleType("android_app", android.ModuleFactoryAdaptor(AndroidAppFactory))