tests: fix refresh tests not stopping fake store for fedora

[sergiocazzolato]

Removing code to avoid stopping the fake store for fedora in refresh
task and adding sanity check before set the refresh

Error:
https://travis-ci.org/snapcore/snapd/builds/257633119

[codecov-io]

Codecov Report

Merging #3629 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3629      +/-   ##
==========================================
- Coverage   75.21%   75.21%   -0.01%     
==========================================
  Files         384      384              
  Lines       33538    33538              
==========================================
- Hits        25226    25224       -2     
- Misses       6491     6493       +2     
  Partials     1821     1821

Impacted Files	Coverage Δ
interfaces/sorting.go	97.43% <0%> (-2.57%)	⬇️

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[zyga]

Looks good, thanks! +1

[niemeyer]

cachio_: I'm finding these prepare and restore in the refresh test pretty hard to follow
17:26:21 cachio_: There are so many exit points.. pretty hard to track what is actually happening there
cachio_: Your changes are minor, so if they fix the problem, LGTM
17:27:23 cachio_: But it would be good to clean that up at some point