corecfg: mock "systemctl" in all corecfg tests

[mvo5]

This is needed to ensure the tests can run on a system that does
not have systemctl (like ubuntu 14.04).

[codecov-io]

Codecov Report

Merging #3854 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3854      +/-   ##
==========================================
+ Coverage   75.71%   75.78%   +0.06%     
==========================================
  Files         413      414       +1     
  Lines       35555    35742     +187     
==========================================
+ Hits        26921    27086     +165     
- Misses       6730     6745      +15     
- Partials     1904     1911       +7

Impacted Files	Coverage Δ
overlord/configstate/config/transaction.go	80.4% <0%> (-4.78%)	⬇️
interfaces/sorting.go	98.71% <0%> (-1.29%)	⬇️
image/image.go	67.06% <0%> (-0.3%)	⬇️
overlord/snapstate/snapstate.go	80.15% <0%> (-0.26%)	⬇️
daemon/api.go	72.39% <0%> (-0.06%)	⬇️
store/store.go	79.82% <0%> (ø)	⬆️
overlord/devicestate/handlers.go	63.31% <0%> (ø)	⬆️
overlord/snapstate/storehelpers.go	80% <0%> (ø)	⬆️
overlord/snapstate/handlers.go	65.1% <0%> (ø)	⬆️
overlord/storestate/storestate.go	100% <0%> (ø)
... and 7 more

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[zyga]

Hmm, I was under the impression that this would never ever touch the real systemctl. I recall seeing a bug (race?) where this test would sometimes fail to figure out something and then proceeded into the "real code" and call systemctl.

While I don't mind the explicit mocking I feel we are missing something here.

[mvo5]

We do touch the real systemd because corecfg runs systemctl --version to detect if the core-support plug is connected. This is where this issue from from. Sorry for not explaining that in code better.

[pedronis]

looks ok, but a doubt

[pedronis]

this looks strange, are we calling here systemctl both directly and through the systemd pkg? or we don't need coreCfgSuite here?

[mvo5]

Indeed, excellent point. This mixing is a mistake that I will fix