test(java): fix, improve and move clean script (#1017)

This includes the following changes:

- move this developer-only script to test/ where it conceptually belongs
  - this also prevents it from being distributed with this package
- fix paths for `android` and `androidx` variants
- make paths relative to the script, not to CWD
- use `removeSync` instead of `existsSync` and `existsSync`
- rename npm script to `clean:java-unit-tests` to clarify scope

bin/clean_test.js

@@ -1,28 +0,0 @@
-/*
-       Licensed to the Apache Software Foundation (ASF) under one
-       or more contributor license agreements.  See the NOTICE file
-       distributed with this work for additional information
-       regarding copyright ownership.  The ASF licenses this file
-       to you 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.
-*/
-
-const fs = require('fs');
-
-if (fs.existsSync('./test/gradlew')) {
-    fs.unlinkSync('./test/gradlew');
-}
-
-if (fs.existsSync('./test/gradlew.bat')) {
-    fs.unlinkSync('./test/gradlew.bat');
-}