Tests: Improve tests for `is_upload_space_available()`.

* Move all tests to a new 'multisite/isUploadSpaceAvailable.php'
* Store original `blog_upload_space` values before the class is loaded.
* Restore these values after each test tear down to avoid pollution.

Commit to testing `is_upload_space_available()` itself by always filtering the amount of space used through `pre_get_space_used`. This allows us to sanely test without worrying about the local environment. In the future, we may be able to remove some of these tests as `get_upload_space_available()` tests are built out and reliable.

See #34037.

Built from https://develop.svn.wordpress.org/trunk@34899


git-svn-id: http://core.svn.wordpress.org/trunk@34864 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-alpha-34898';
+$wp_version = '4.4-alpha-34899';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.