tests: avoid timing issues when checking cached log contents

beaker-watchdog could still be writing to the cached log file when the
tests check it for expected data, leading to intermittent failures like
this:

Traceback (most recent call last):
  File "IntegrationTests/src/bkr/inttest/labcontroller/test_watchdog.py", line 62, in test_console_log_not_recreated_after_removed
    self.assertEquals(open(self.cached_console_log, 'r').read(), existing_data)
AssertionError: '' != 'Existing data\n'

Change-Id: I3fb5dba3842e61e107e1ed22aad9888f587922de
Merge "tests: avoid timing issues when checking cached log contents" into develop