testlib.agents: disable 'shell=True'.

This is causing flake in the tests.
`subprocess` methods expect command as a `str` when `shell=True`, otherwise a `list[str]`.


Change-Id: I23eb74b0c950d14dd53b4c0d0f8f597ad7cfb30d

tests/testlib/agent.py

@@ -294,7 +294,7 @@ def wait_for_baking_job(central_site: Site, expected_start_time: float) -> None:
 def _remove_omd_status_cache() -> None:
  logger.info("Removing omd status agent cache...")
  try:
- run(["rm", "-f", str(OMD_STATUS_CACHE)], shell=True, sudo=True)
+ run(["rm", "-f", str(OMD_STATUS_CACHE)], sudo=True)
  except subprocess.CalledProcessError as excp:
  excp.add_note("Failed to remove agent cache!")
  raise excp