Create fixtures to ensure temporary directories are deleted after use, introduced by PR #1155

[eddiebergman]

PR #1155 limits the lifetime of the backend directories by deleting it after fit() is finished.

However, some tests tests preserve these directories with delete_tmp_folder_after_terminate=False, having autosklearn create random directories which can then not be manually cleaned up.

The easiest solution I can see is to rely on the tmpdir fixture provided by pytest and pass that to the backend creation. This way, upon the end of the test, pytest will clean up the directories it created for the test.

[eddiebergman]

This would be fixed by solving issue #1305 so this is being closed.