Windows ZipArchive::close() errors with PHPUnit

These errors have been detected recently under windows and are consistently reproducible with Moodle 3.10 and master.

• travis: https://travis-ci.com/github/stronk7/moodle/jobs/400704270
• locally:

  $ vendor/bin/phpunit --filter moodle_content_writer_test::test_export_user_preference_context_course

  PHPUnit test environment setup complete.

  Moodle 3.10dev+ (Build: 20201009), 69959c371f74398b2a44893f19f6a3af2534e1b2

  Php: 7.2.12, pgsql: 10.6, OS: Windows NT 10.0 AMD64

  PHPUnit 7.5.20 by Sebastian Bergmann and contributors.

  ...EEE 6 / 6 (100%)

  Time: 11.42 seconds, Memory: 464.00 MB

  There were 3 errors:

  1) moodle_content_writer_test::test_export_user_preference_context_course with data set "basic" ('core_privacy', 'onekey', 'value', 'description')

  ZipArchive::close(): Read error: No such file or directory

  C:\Users\stronk7\git_moodle\integration\lib\filestorage\zip_archive.php:225

  C:\Users\stronk7\git_moodle\integration\lib\filestorage\zip_packer.php:154

  C:\Users\stronk7\git_moodle\integration\privacy\classes\local\request\moodle_content_writer.php:694

  C:\Users\stronk7\git_moodle\integration\privacy\tests\moodle_content_writer_test.php:1225

  C:\Users\stronk7\git_moodle\integration\privacy\tests\moodle_content_writer_test.php:618

  C:\Users\stronk7\git_moodle\integration\lib\phpunit\classes\advanced_testcase.php:80

  ...

  ...

Also, it has been noticed (along this issue research) that all phpunit executions (and maybe others) are throwing some PHP Warnings with unlink() trying to remove non-existing directory:

Warning: unlink(C:\Users\travis\roots\phpunitdata/_temp_2a60d2cfee1b184340eb76f43aeebba6/_temp_268c851d269a7830e5e6cb615777eb41/_temp_951be5e0378d9543d31b82c6ca468653/_temp_9c100731e36c8ad6f88291ae16b5faa4/_temp_0e223fb4008ddcec3a00d1e182116e19/bee124ad3a85b27799e56ad58e029cccf68b783b.cache): No such file or directory in C:\Users\travis\build\stronk7\moodle\lib\moodlelib.php on line 9878

Reproducible with any OS.

So this issue is about to try to fix both issues, both related/regressions of MDL-66928:

1) Reduce unnecessary path length while keeping uniqueness guaranteed.
2) Check and fix why those warnings are happening at the end of the execution, consistently.