REST API plugins endpoint create item tests don't work in all environments

[TimothyBlynJacobs]

A couple of hosts that are running the WordPress test suite reporter are encountering issues with the create_item tests for the new plugins endpoint introduced in [48242]. See ​wordpress/phpunit-test-reporter#83 for details.

WP_REST_Plugins_Controller_Test::test_create_item
unlink(/var/www/vhosts/wp-test-runner/tests/phpunit/includes/../data/link-manager.zip): No such file or directory
WP_REST_Plugins_Controller_Test::test_create_item_and_activate
unlink(/var/www/vhosts/wp-test-runner/tests/phpunit/includes/../data/link-manager.zip): No such file or directory
WP_REST_Plugins_Controller_Test::test_create_item_and_activate_errors_if_no_permission_to_activate_plugin

I believe this is due to the language pack upgrader running after the plugin has been installed. We should make the package override more specific and possibly unhook the language upgrader async check entirely.

[TimothyBlynJacobs]

In 48524:

REST API: Make plugin installation tests more robust on alternate test environments.

The REST API plugin installation tests use the upgrader_pre_download filter to avoid downloading the test plugin from WordPress.org. Previously, this would apply to any upgrader, which caused issues if the testing environment required a language update.

Now, the filter only overwrites the file if the Plugin_Upgrader is being used which should hopefully prevent the issue.

Props pfefferle, TimothyBlynJacobs.
Fixes #50671.