interfaces,tests: skip unknown plug/slot interfaces (2.29)

[zyga]

This patch changes the Repository.AddSnap method to skip unknown
interfaces when adding constituent plugs and slots. This regression
happened because the repository no longer validates the snap, assuming
the loader did. The loader did notice but did not remove the offending
interfaces.

As it is unclear what should happen, this patch implements a minimal
solution to prevent snapd from crashing on nil interface.

Fixes: https://bugs.launchpad.net/snappy/+bug/1732555
Signed-off-by: Zygmunt Krynicki zygmunt.krynicki@canonical.com

[codecov-io]

Codecov Report

Merging #4228 into release/2.29 will increase coverage by 0.02%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           release/2.29   #4228      +/-   ##
===============================================
+ Coverage         75.58%   75.6%   +0.02%     
===============================================
  Files               433     433              
  Lines             37266   37270       +4     
===============================================
+ Hits              28166   28177      +11     
+ Misses             7128    7122       -6     
+ Partials           1972    1971       -1

Impacted Files	Coverage Δ
interfaces/repo.go	97.66% <100%> (+0.01%)	⬆️
cmd/snap-seccomp/main.go	54.21% <0%> (+2.81%)	⬆️

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