Removing order dependency of test_remove_bookmark`

[sturmianseq]

This PR aims to improve test reliability by removing order dependency of test_remove_bookmark, by calling add_bookmark before executing remove_bookmark.

The test would fail if test_remove_bookmark is run ahead of test_add_bookmark, running commands pip install pytest-random-order; pytest --random-order tests/test_marlin.py:

>       mock_object.remove_bookmark()
tests/test_marlin.py:36: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <marlin.manage.ManageBookmark object at 0x7fa8a5f29580>

    def remove_bookmark(self):
        os.chdir(self.m_path)
>       os.unlink(self.bookmark_name)
E       FileNotFoundError: [Errno 2] No such file or directory: 'mock'