[Data Liberation] Add support for terms meta and new unit tests

[zaerl]

Motivation for the change, related issues

This code is cherry-picked from #2030. This is done to publish those parts during this discussion: #2090.

Implementation details

New unit tests

1. Added all the ~130 WordPress core unit tests. Some of them are marked as skipped waiting for [Data Liberation] Topological sorter, entities remapping and add missing imports #2030 to be merged
2. Updated the WXRs to the latest version.
3. Added a PlaygroundTestCase base class that clean up the WordPress database after a test, as _delete_all_data does.

New meta

1. Added support for terms meta.
2. Fixed wrong meta_{key|value} strings.

New PHPUnit filter

This PR adds a PHPUNIT_FILTER constant to packages/playground/data-liberation/tests/import/blueprint-import.json. If the value is not falsy it will be passed to PHPUnit when calling npx nx run playground-data-liberation:test:wp-phpunit. So, for example, you can set "PHPUNIT_FILTER": "WPRewriteUrlsTests". It will be the same as running phpunit --filter WPRewriteUrlsTests.

Testing Instructions (or ideally a Blueprint)

Unit tests

npx nx run playground-data-liberation:test:wp-phpunit

# Or only tests that do not need a WordPress environment
cd packages/playground/data-liberation
./vendor/bin/phpunit

Many tests can be run only in a real WordPress environment, which is why wp-phpunit exists.