Update test_update.py to use mock_update_handler

[narrieta]

The UpdateTests exercise the code using UpdateHandler.run(), which starts all the background threads. This leads to random errors because these threads can create extra files, log messages, telemetry events, etc.

The mock_update_handler does not start the background threads.

[codecov]

Codecov Report

Merging #2547 (6274ec5) into develop (e09d449) will decrease coverage by 0.35%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2547      +/-   ##
===========================================
- Coverage    72.11%   71.76%   -0.36%     
===========================================
  Files          102      102              
  Lines        15314    15314              
  Branches      2426     2426              
===========================================
- Hits         11044    10990      -54     
- Misses        3783     3834      +51     
- Partials       487      490       +3     

Impacted Files	Coverage Δ
azurelinuxagent/ga/collect_telemetry_events.py	80.13% <0.00%> (-11.99%)	⬇️
azurelinuxagent/ga/env.py	51.74% <0.00%> (-9.80%)	⬇️
azurelinuxagent/common/interfaces.py	57.14% <0.00%> (-7.15%)	⬇️
azurelinuxagent/ga/send_telemetry_events.py	92.68% <0.00%> (-2.44%)	⬇️
azurelinuxagent/ga/monitor.py	74.41% <0.00%> (-1.75%)	⬇️
azurelinuxagent/ga/remoteaccess.py	88.42% <0.00%> (-1.06%)	⬇️
azurelinuxagent/common/event.py	85.36% <0.00%> (-0.68%)	⬇️
azurelinuxagent/ga/update.py	88.65% <0.00%> (+0.47%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e09d449...6274ec5. Read the comment docs.