fix: allow logs to propagate upstream for caplog testing

[ohmayr]

When tests are run in parallel and a base logger is not configured, then the log propagation from the base logger (i.e. a google named logger) to the root logger is disabled (this happens during client construction):

logger = logging.getLogger("google")
logger.propagate = False

Given that the above is a global config, any of the tests that use the caplog fixture start to fail once the propagation of logs to the root logger is disabled. This means that nothing is captured for a logger (by caplog) if it does not propagate to the root logger. See pytest-dev/pytest#3697.

As a workaround, we can allow propagation of logs to the root logger for a test run and then revert it to the original value once the test completes.

Fixes: b/388826364

Note: By default, log propagation to the root logger is disabled. This behaviour is captured in the README.