Fix indentation in clean utility

`utils.clean` extracts a test method content re-indenting the function for whitespaces.
This commit fixes to issues with indentation

* windows line feed `\r\n`
* tab indentation `\t`

I've added a test inside `test/acceptance`, it should run with `make test-unit`, and I've tried to follow your coding style (changes are very small)
Hope you'd like it :smile: