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.