tests: fix for upgrade test when it is repeated

[sergiocazzolato]

When the upgrade test is repeated using -repeat option, it is failing
due to snapd it is not being purged for each task.

It is happening that the prev version is the same than the new one:

• prevsnapdver='snapd 1337.2.26.3'
• snapdver='snapd 1337.2.26.3'
• '[' 'snapd 1337.2.26.3' '!=' 'snapd 1337.2.26.3' ']'

[codecov-io]

Codecov Report

Merging #3405 into master will increase coverage by 0.94%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3405      +/-   ##
==========================================
+ Coverage   77.49%   78.44%   +0.94%     
==========================================
  Files         369      372       +3     
  Lines       25472    26659    +1187     
==========================================
+ Hits        19740    20912    +1172     
+ Misses       3980     3975       -5     
- Partials     1752     1772      +20

Impacted Files	Coverage Δ
store/errors.go	0% <0%> (ø)	⬆️
interfaces/builtin/mount_observe.go	100% <0%> (ø)	⬆️
cmd/snap-repair/main.go	50% <0%> (ø)
cmd/snap-repair/cmd_run.go	80% <0%> (ø)
httputil/retry.go	93.22% <0%> (ø)
overlord/snapstate/storehelpers.go	86.36% <0%> (+1.36%)	⬆️
cmd/snap/cmd_aliases.go	96% <0%> (+2%)	⬆️
store/auth.go	90.2% <0%> (+2.56%)	⬆️
interfaces/sorting.go	96.66% <0%> (+3.33%)	⬆️
overlord/ifacestate/helpers.go	67.45% <0%> (+4.58%)	⬆️
... and 7 more

---

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 77d61da...a28f934. Read the comment docs.

[zyga]

+1