tests: remove import path mangling while importing 'test._common'

The import path mangling is not relevant (anymore?) for the two
ways of running tests:

* `python3 test/testall.py` (see CONTRIBUTING.rst):
  The `testall.py` script already adds the project path to `sys.path`.
* `tox -e py-cov`: this command is supposed to be run from the project
  path. Thus, the current directory is already the first of location
  in `sys.path`.

The previous mangling of the import path while loading a module could
lead to unwanted side-effects hidden in an unexpected location.
Instead, import path mangling should take place in the script being
called by the user (here: `testall.py`).

test/_common.py

@@ -22,8 +22,6 @@ import time
 import unittest
 from contextlib import contextmanager
 
-# Mangle the search path to include the beets sources.
-sys.path.insert(0, "..")
 import beets  # noqa: E402
 import beets.library  # noqa: E402