fix(test): avoid racy reader vs writer contender in test_rw_lock

[StephenSorriaux]

See test failure https://github.com/python-zk/kazoo/actions/runs/8116444130/job/22206401448?pr=744

        for contender in ("reader", "writer"):
            thread, event = contender_bits[contender]
    
            with self.condition:
                while not self.active_thread:
                    self.condition.wait()
>               assert self.active_thread == contender
E               AssertionError: assert 'writer' == 'reader'
E                 - reader
E                 + writer

kazoo/tests/test_lock.py:515: AssertionError

Why is this needed?

test_rw_lock is creating 2 threads (reader and writer) and, after being started, it is expected that reader is a contender before writer. In some busy systems (like the CI... it's always the CI!) this may not be true and lead to the test failure because writer can be a contender before reader. This commit makes sure that reader is always a contender before writer.

Proposed Changes

• wait for reader being a contender before starting writer
• rewrote some minimal part of the test, just because I was there

Does this PR introduce any breaking change?

No!

[jeffwidman]

Makes sense to me. Nice cleanup.

[ceache]

Looks good to me!