Improve systemd tests and reset stdout/stderr

[pgombar]

Address leftover comments from #1556:

• improve systemd tests
• reset stdout/stderr before invoking another subprocess
• add unit tests for azurelinuxagent.utils.processutil.read_output
• add unit tests for logging when calling azurelinuxagent.common.event.add_event

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This change is 

[codecov]

Codecov Report

Merging #1563 into develop will decrease coverage by <.01%.
The diff coverage is 33.33%.

@@             Coverage Diff             @@
##           develop    #1563      +/-   ##
===========================================
- Coverage    63.38%   63.38%   -0.01%     
===========================================
  Files           78       78              
  Lines        11056    11057       +1     
  Branches      1559     1559              
===========================================
  Hits          7008     7008              
- Misses        3709     3710       +1     
  Partials       339      339

Impacted Files	Coverage Δ
azurelinuxagent/common/cgroupapi.py	62.5% <0%> (-0.28%)	⬇️
azurelinuxagent/common/utils/processutil.py	100% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #1563 into develop will decrease coverage by <.01%.
The diff coverage is 33.33%.

@@             Coverage Diff             @@
##           develop    #1563      +/-   ##
===========================================
- Coverage    63.38%   63.38%   -0.01%     
===========================================
  Files           78       78              
  Lines        11056    11057       +1     
  Branches      1559     1559              
===========================================
  Hits          7008     7008              
- Misses        3709     3710       +1     
  Partials       339      339

Impacted Files	Coverage Δ
azurelinuxagent/common/cgroupapi.py	62.5% <0%> (-0.28%)	⬇️
azurelinuxagent/common/utils/processutil.py	100% <100%> (ø)	⬆️

---

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 8761775...42c2570. Read the comment docs.

[vrdmr]

We might need to add new tests in test_extension.py, with an implementation in TestExtensionWithCGroupsEnabled. Essentially making the test_ext_handler_with_systemd_cgroup_enabled work. When looking at the coverage of the SystemdCgroupAPI, I don't see that the tests you added are still exercising the code in there.

[pgombar]

We might need to add new tests in test_extension.py, with an implementation in TestExtensionWithCGroupsEnabled. Essentially making the test_ext_handler_with_systemd_cgroup_enabled work. When looking at the coverage of the SystemdCgroupAPI, I don't see that the tests you added are still exercising the code in there.

I agree, but this PR is contained to improving tests around systemd-run, not against the whole class. I can do that in a separate PR since it will be a lot of changes.

The issue with test_ext_handler_with_systemd_cgroup_enabled not working is purely because Travis doesn't support cgroups/systemd. When ran locally, the test passes. For the code coverage you linked, a better candidate for testing this would be LaunchCommandTestCase.

[vrdmr]

LGTM