Build/Test Tools: Improve messaging when PHPUnit Polyfills cannot be found.

Previously, two situations were taken in to account:
1. The `WP_TESTS_PHPUNIT_POLYFILLS_PATH` constant is defined => show message specific to that constant not being set correctly.
    This message would typically be shown for plugin/theme integration tests which are already aware of the changes in WP 5.9.
2. The constant is not defined => show a message to run `composer update`.
    This message is intended for people trying to run the WP Core tests.

This left two situations unaccounted for:
- Someone trying to run the WP Core tests, but not having set the `WP_RUN_CORE_TESTS` constant or not having set it to `1`.
- Someone trying to run plugin/theme integration tests without the new `WP_TESTS_PHPUNIT_POLYFILLS_PATH` constant being defined as they are not (yet) aware of the changes made in WP 5.9.

The changes made in this commit, are intended to improve the error messages displayed in those situations.

Follow-up to [51598], [51810].

Props jrf, schlessera, hellofromTonya, jeherve, lucatume.
See #46149.
Built from https://develop.svn.wordpress.org/trunk@51811


git-svn-id: http://core.svn.wordpress.org/trunk@51418 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51810';
+$wp_version = '5.9-alpha-51811';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.