Build/Test Tools: Check if Travis/GitHub Actions environment variables are defined.

This adjusts the logic for determining whether to skip some tests when not in the primary branch, and allows for running these tests locally.

Follow-up to [47000], [47001], [49264], [49267], [49280].

See #50401.
Built from https://develop.svn.wordpress.org/trunk@49916


git-svn-id: http://core.svn.wordpress.org/trunk@49615 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-alpha-49915';
+$wp_version = '5.7-alpha-49916';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.