interfaces: don't udev tag devmode or classic snaps (2.29)

[jdstrand]

devmode and classic snaps are meant to run without confinement, but the udev
backend unconditionally udev tags devices resulting in permission denied errors
when for devices not in connected interfaces. While classic snaps currently
aren't allowed to plugs interfaces, this is expected to change so fix this now
too.

Reference:
https://forum.snapcraft.io/t/device-cgroup-is-applied-to-devmode-snap/2663

While we could simply not generate the files in /etc/udev/rules.d when in
devmode or classic, we instead generate those files with commented out the
udev rules. This makes the testsuite changes much smaller, but also allows
someone working in devmode to see what would be done, and to adjust the file in
/etc/udev/rules.d as needed when developing their snap.

This adds four spread tests: devmode, classic, jailmode and strict mode. While
much of the cgroup handling is done in tests/main/security-device-cgroups,
these tests are more about high-level blackbox testing in the manner a
developer would experience things.

[zyga]

LGTM

[codecov-io]

Codecov Report

Merging #4131 into release/2.29 will increase coverage by <.01%.
The diff coverage is 100%.

@@               Coverage Diff                @@
##           release/2.29    #4131      +/-   ##
================================================
+ Coverage         75.79%   75.79%   +<.01%     
================================================
  Files               433      433              
  Lines             37247    37253       +6     
================================================
+ Hits              28230    28236       +6     
  Misses             7044     7044              
  Partials           1973     1973

Impacted Files	Coverage Δ
interfaces/udev/backend.go	79.45% <100%> (+1.84%)	⬆️

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