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

test/clean.js

@@ -0,0 +1,32 @@
+/*
+       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-extra');
+const path = require('path');
+
+/**
+ * This script is to be run manually (e.g. by npm run clean:java-unit-tests) if
+ * you want to upgrade gradlew or test its proper generation.
+ */
+
+for (const variant of ['android', 'androidx']) {
+    for (const file of ['gradlew', 'gradlew.bat']) {
+        fs.removeSync(path.join(__dirname, variant, file));
+    }
+}