tests: use dnf --refresh install to avert stale cache

[zyga]

While our package abstraction helper for spread tests does call dnf
makecache evidence in test failures shows that something is not quite
right and we fail to install packages that have since, in the archive,
been upgraded to a new revision and we attempt to install the old
version oblivious to that fact.

Neal Gompa suggested that we use the --refresh option on dnf install to
always ensure that the package cache is updated if it is out of date.

Signed-off-by: Zygmunt Krynicki zygmunt.krynicki@canonical.com

[mvo5]

\o/ if it helps …

[codecov-io]

Codecov Report

Merging #3721 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3721      +/-   ##
==========================================
- Coverage   75.65%   75.65%   -0.01%     
==========================================
  Files         391      391              
  Lines       33849    33849              
==========================================
- Hits        25609    25608       -1     
- Misses       6418     6419       +1     
  Partials     1822     1822

Impacted Files	Coverage Δ
cmd/snap/cmd_aliases.go	93.33% <0%> (-1.67%)	⬇️
overlord/snapstate/snapstate.go	80.45% <0%> (-0.27%)	⬇️
wrappers/binaries.go	79.31% <0%> (+10.34%)	⬆️

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