Replace the deprecated pytest.yield_fixture in test/test_tls.py

[jaysonsantos]

Found while working on #274.

Evidence

test/test_tls.py:16 uses pytest.yield_fixture:

@pytest.yield_fixture(scope="module", autouse=True)
def memcached_tls():

pytest.yield_fixture is a deprecated alias of @pytest.fixture. The suite
prints one PytestDeprecationWarning for it on every run.

#274 replaced the four uses in test/conftest.py. It did not touch
test/test_tls.py, because that file is outside its stated scope.

Scope

One line. Replace @pytest.yield_fixture with @pytest.fixture.

Consider whether test/test_tls.py should also use the shared
_wait_until_accepting helper that #274 added to test/conftest.py. It holds
its own time.sleep(0.1) and its own p.poll() check today.

Acceptance criteria

• test/test_tls.py uses @pytest.fixture.
• pytest -q prints no PytestDeprecationWarning.
• The TLS tests still pass and still skip cleanly on a memcached without
  TLS support.

Order

Do #274 first. It adds the helper this issue can reuse.