Windows unit tests failures caused by backup controllers not being destroyed

With latest roll (May 19th 2020), windows unit tests are back to fail with (legion of) errors like this:

1) core_backup_async_helper_testcase::test_get_backup_record

1235unlink(C:\Users\travis\roots\phpunitdata/temp/backup/e672a7b14886295d7b069afc7892e5f3.log): Resource temporarily unavailable

Link: https://travis-ci.org/github/stronk7/moodle/builds/688922236

That uses to happen when some backup or restore controller in tests is missing its corresponding ->destroy() call, usually in charge of closing file handlers, destroying circular structures and other bits. Under Windows, this is specially noticeably because it locks those files completely (while under Unix, everything passes ok).

This is about to find the test / code missing the ->destroy() calls and add them, ensuring we are back to green with Windows.