tests: remove obsolete snap and external tests

[sergiusens]

Signed-off-by: Sergio Schvezov sergio.schvezov@canonical.com

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[zyga]

Less code, what is not to like!

[codecov-io]

Codecov Report

Merging #2421 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2421      +/-   ##
==========================================
- Coverage    90.1%   90.09%   -0.02%     
==========================================
  Files         196      196              
  Lines       12891    12886       -5     
  Branches     1956     1956              
==========================================
- Hits        11615    11609       -6     
- Misses        876      877       +1     
  Partials      400      400

Impacted Files	Coverage Δ
snapcraft/internal/pluginhandler/_plugin_loader.py	82.65% <0%> (-2.05%)	⬇️
snapcraft/plugins/nodejs.py	90.25% <0%> (-0.31%)	⬇️
snapcraft/internal/elf.py	80.82% <0%> (+0.34%)	⬆️

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[sergiusens]

unrelated plainbox failures, merging.

[kyrofa]

This broke so many docs :'( .