It has been impossible to write php unit integration tests with code that calls file_stoarge.php 
create_file directly or indirectly since this commit:
72b43c3541e3ce2ff7e88d59490396a5b4bbcb79

I maintain a plugin (tool_ally). We have php unit tests to ensure a custom hook is called when creating files for different module types. This now fails because the create_file function aborts early if phpunit is running.

```
if (defined('PHPUNIT_TEST') && PHPUNIT_TEST)

```

There are two possible fixes I would like to propose:

1) Allow for create_file to be called in a way that the phpunit skip code condition is bypassed

2) Modify file_storage to being a moodle class where it works with dependency injection so that an alternative file_storage class can be used.

I'm happy to make this change and file a PR but would like to know which is the preferred fix.