tests: mark interfaces-openvswitch as manual due to prepare errors

[fgimenez]

We are still getting eventual errors on tests/main/interfaces-openvswitch, like http://paste.ubuntu.com/24856387/

[codecov-io]

Codecov Report

Merging #3479 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3479      +/-   ##
==========================================
- Coverage   77.16%   77.16%   -0.01%     
==========================================
  Files         373      373              
  Lines       25793    25793              
==========================================
- Hits        19904    19902       -2     
- Misses       4132     4134       +2     
  Partials     1757     1757

Impacted Files	Coverage Δ
interfaces/sorting.go	93.33% <0%> (-3.34%)	⬇️
cmd/snap/cmd_aliases.go	94% <0%> (-2%)	⬇️
overlord/ifacestate/helpers.go	65.54% <0%> (ø)	⬆️

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[zyga]

+1

[morphis]

LGTM, please lets get this in!