snap: add new `snap pack` and use in tests

[mvo5]

No description provided.

[codecov-io]

Codecov Report

Merging #3946 into master will increase coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #3946      +/-   ##
==========================================
+ Coverage   75.91%   75.95%   +0.04%     
==========================================
  Files         418      420       +2     
  Lines       36272    36284      +12     
==========================================
+ Hits        27536    27560      +24     
+ Misses       6805     6796       -9     
+ Partials     1931     1928       -3

Impacted Files	Coverage Δ
snap/snaptest/snaptest.go	50.64% <0%> (ø)	⬆️
snap/pack/pack.go	73.68% <100%> (ø)
httputil/redirect18.go	100% <0%> (ø)
httputil/transport17.go	100% <0%> (ø)
overlord/snapstate/snapstate.go	80.09% <0%> (+0.24%)	⬆️
overlord/ifacestate/helpers.go	63% <0%> (+0.66%)	⬆️
cmd/snap/cmd_aliases.go	95% <0%> (+1.66%)	⬆️
wrappers/binaries.go	79.54% <0%> (+6.81%)	⬆️
httputil/useragent.go	85% <0%> (+7.5%)	⬆️

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