Build/Test Tools: Improve messaging when PHPUnit Polyfills do not comply with version requirements.

Previously, two situations were taken in to account:
1. The `WP_TESTS_PHPUNIT_POLYFILLS_PATH` constant is defined => just show a message about the version mismatch.
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 could lead to an unclear situation for people trying to run plugin/theme integration tests without the new `WP_TESTS_PHPUNIT_POLYFILLS_PATH` constant being defined.

They could be shown the message to run `composer update` while if they would do so for their local install without adding the Polyfills, the message would still display the next time they would attempt to run the tests.

This commit:
1. Provides more information about the PHPUnit Polyfills version detected vs the version expected.
2. Shows a more specific message to guide users which have the `WP_TESTS_PHPUNIT_POLYFILLS_PATH` constant declared.
3. Only shows the message to run `composer update` when the `WP_RUN_CORE_TESTS` constant is declared to prevent confusing people more.

Follow-up to [51598], [51810], [51811].

Props jrf, schlessera, hellofromTonya, jeherve, lucatume.
See #46149.
Built from https://develop.svn.wordpress.org/trunk@51812


git-svn-id: http://core.svn.wordpress.org/trunk@51419 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51811';
+$wp_version = '5.9-alpha-51812';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.