tests: api: fix intertest dependency on repository locking

In test classes based on unittest, tests are executed in alphabetical order.
In test classes based on pytest, tests are executed in the order they are
specified. This difference revealed a problem in the API tests:

- test_api_lock_repo_lock_optional_locked locks the test repository
- test_api_get_locks_regular_user gets the current locks and expects it to
be empty

With unittest as base class, this worked fine because the 'get_locks' group
of tests are executed before the 'lock_repo' group (alphabetical order).
Using a real pytest-based test class, the order is swapped and the locked
repository from the first test invalidates the preconditions of the second
test.

Fix this specific problem by releasing the lock from
test_api_lock_repo_lock_optional_locked.

This commit does not fix other interdependencies between tests. For example,
test_api_lock_repo_lock_optional_locked expects the existing lock state to
be 'locked' but did not lock the repo itself; instead it expects a previous
test to have locked. In practice, this is
test_api_lock_repo_lock_aquire_optional_userid.
A full solution would make each test fully self contained so that tests can
be executed in random order. The pytest extension pytest-random can help
detecting these problems.