Tests: Improve bootstrap error message for when `ABSPATH` folder does not exist.

The PHPUnit tests are/should generally be run on the `src` directory, so changes just made can be tested. While testing via the `build` directory is also still supported, this is not the default.

This was last changed in [50441], but that commit did not remove/update the error message thrown by the test bootstrap file.

This last change also did not take into account that that change meant that people had to update their own `wp`tests-config.php` file to match the change made in Core and would otherwise still get the outdated error message.

This commit changes the messaging in the test bootstrap file to be more in line with the current reality, while still accounting for the fact that tests can be run from both `src` as well as `build`.

Follow-up to [49569], [50441], [51581].
Props jrf, hellofromTonya.
Built from https://develop.svn.wordpress.org/trunk@51669


git-svn-id: http://core.svn.wordpress.org/trunk@51275 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51668';
+$wp_version = '5.9-alpha-51669';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.