tests/lib: use mktemp instead of tempfile to work cross-distro

[morphis]

No description provided.

[codecov-io]

Codecov Report

Merging #3359 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3359      +/-   ##
==========================================
- Coverage   77.55%   77.55%   -0.01%     
==========================================
  Files         366      366              
  Lines       25126    25146      +20     
==========================================
+ Hits        19486    19501      +15     
- Misses       3894     3897       +3     
- Partials     1746     1748       +2

Impacted Files	Coverage Δ
interfaces/core.go	95.45% <0%> (-4.55%)	⬇️
interfaces/builtin/common.go	55.55% <0%> (-3.97%)	⬇️
interfaces/sorting.go	90% <0%> (-3.34%)	⬇️
cmd/snap/cmd_interfaces.go	84.9% <0%> (-2.85%)	⬇️
interfaces/builtin/account_control.go	100% <0%> (ø)	⬆️
overlord/ifacestate/helpers.go	62.86% <0%> (ø)	⬆️
interfaces/builtin/alsa.go	100% <0%> (ø)	⬆️
client/interfaces.go	90% <0%> (ø)	⬆️
interfaces/builtin/network.go	100% <0%> (ø)	⬆️
interfaces/repo.go	97.65% <0%> (+0.03%)	⬆️
... and 2 more

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