tests: fix for the test postrm-purge

[sergiocazzolato]

The fix is moves the clean up of the systemd service unit config files
to the reset.

This change will address the scenarios/tests where snapd is not in a
working state in the restore step.

Error fixing:
https://travis-ci.org/snapcore/snapd/builds/240775990

[codecov-io]

Codecov Report

Merging #3448 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3448      +/-   ##
==========================================
- Coverage   77.23%   77.22%   -0.02%     
==========================================
  Files         373      373              
  Lines       25645    25645              
==========================================
- Hits        19808    19805       -3     
- Misses       4087     4089       +2     
- Partials     1750     1751       +1

Impacted Files	Coverage Δ
interfaces/sorting.go	90% <0%> (-6.67%)	⬇️
overlord/snapstate/snapstate.go	81.26% <0%> (-0.24%)	⬇️
cmd/snap/cmd_aliases.go	96% <0%> (+2%)	⬆️

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[chipaca]

Thank you for this!

As this fixes master, and only changes the tests, I think it's OK to land with just one +1.

One thing I'd like to point out is that we've discussed in the past that our git logs aren't very good, and we should try to improve them. One way I found that helped me visualise this, and then improve, is to run git log --oneline --no-merges (or, if you're feeling fancy, git log --pretty="format:%C(auto)%h %C(dim)%<(16,trunc)%an %C(nodim)%s" --no-merges). Also perhaps relevant is https://chris.beams.io/posts/git-commit/.

[sergiocazzolato]

@chipaca thanks for pointing that, I'll work on this.