PYTHON-4609 Speed up unified tests

[ShaneHarvey]

PYTHON-4609

Explanation: The legacy test runner has this code:

class SpecRunner(IntegrationTest):
...
    @classmethod
    def _setup_class(cls):
...
        # Speed up the tests by decreasing the heartbeat frequency.
        cls.knobs = client_knobs(heartbeat_frequency=0.1, min_heartbeat_interval=0.1)
        cls.knobs.enable()

[ShaneHarvey]

Tests are running quickly again, actually even a few minutes quicker than before.

[ShaneHarvey]

Note the test failures are due to other issues, like: #1769