test(alerter): fix time and timezone issues in alerter tests

Fixes #1330

Some tests wouldn't pass locally if `TZ` was unset, or not set to to,
e.g., UTC.

One issue was the fallback to `"local"` if `TZ` was unset - this is a
valid value internally within simplemonitor, but I don't think valid as
a time zone.

Also, having many tests use the local timezone creates some
unpredictability in cases where there are hardcoded offsets, or where
the test depends on the local timezone having a specific offset from
UTC.

Mock `TZ` as `UTC` or `MST` explicitly, and adjust existing tests based
on offsets from that where needed.

Fix / unskip some skipped tests

setup.cfg

@@ -11,7 +11,7 @@ per-file-ignores =
     simplemonitor/__init__.py:F401
 
 [isort]
-known_third_party = arrow,boto3,botocore,freezegun,importlib_metadata,jinja2,markupsafe,oauthlib,paho,paramiko,psutil,pyaarlo,pytest,requests,ring_doorbell,sphinx,sphinx_rtd_theme,twilio,win32event,win32service,win32serviceutil
+known_third_party = arrow,boto3,botocore,freezegun,importlib_metadata,jinja2,markupsafe,oauthlib,paho,paramiko,psutil,pyaarlo,requests,ring_doorbell,sphinx,sphinx_rtd_theme,twilio,win32event,win32service,win32serviceutil
 known_first_party = Alerters,Loggers,Monitors,envconfig,util,simplemonitor
 line_length=88
 multi_line_output=3