Build/Test Tools: Install WordPress Importer plugin when installing the Docker-based local environment.

The WordPress Importer plugin is now downloaded to the `tests/phpunit/data/plugins` directory when running `npm run env:install`.

This ensures that the PHPUnit test suite will not fail when the plugin is missing.

This also introduces a new `WP_IMPORTER_REVISION` variable to the `.env` file, to control the SVN revision that is checked out.

Props johnbillion.
Fixes #49720.
Built from https://develop.svn.wordpress.org/trunk@50285


git-svn-id: http://core.svn.wordpress.org/trunk@49930 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-beta2-50284';
+$wp_version = '5.7-beta2-50285';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.