Let tests override environment

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

Test: none
Change-Id: I683cd9c9e0db61c9bfd2adb27fca78f558f225c4

python/python_test.go

@@ -450,7 +450,7 @@ func setupBuildEnv(t *testing.T) (config android.Config, buildDir string) {
 		t.Fatal(err)
 	}
 
-	config = android.TestConfig(buildDir)
+	config = android.TestConfig(buildDir, nil)
 
 	return
 }