Fix slow halt reboot test

[jkmar]

The test_execute_reboot_fail_halt_timeout test was very slow - it took around a minute:

user@host:/sonic/src/sonic-host-services$ pytest-3 tests/host_modules/reboot_test.py::TestReboot::test_execute_reboot_fail_halt_timeout
=============================================================== test session starts ================================================================
platform linux -- Python 3.9.2, pytest-6.0.2, py-1.10.0, pluggy-0.13.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /sonic/src/sonic-host-services, configfile: pytest.ini
plugins: pyfakefs-5.2.4, cov-2.10.1
collected 1 item

tests/host_modules/reboot_test.py::TestReboot::test_execute_reboot_fail_halt_timeout PASSED                                                  [100%]
...
===================================================== 1 passed, 3 warnings in 60.44s (0:01:00) =====================================================

The reason is that within this while loop: https://github.com/sonic-net/sonic-host-services/blob/master/host_modules/reboot.py#L131, only sleep in the body was mocked, but the condition wasn't, which caused the loop to be run continuously until the halt timer expired.

This PR fixes the issue, by mocking the monotonic clock used in the loop's condition, so that it ends earlier.
Now the test takes much less time:

user@host:/sonic/src/sonic-host-services$ pytest-3 tests/host_modules/reboot_test.py::TestReboot::test_execute_reboot_fail_halt_timeout
...
========================================================== 1 passed, 3 warnings in 0.46s ===========================================================

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jkmar]

Please review @kwangsuk @anand-kumar-subramanian @mbalachandar @amrutasali @sachinholla @sneelam20

[anders-nexthop]

This change looks fine to me, and the test is passing. @jkmar you will have better luck getting visibility into PRs if you create a corresponding issue for them; individual PRs do not get looked at very often if at all, whereas issues are generally triaged every couple of weeks.