tests: restart rng-tools services after few seconds

[sergiocazzolato]

Sometimes the service is not starting properly when it is restarted
inmediatly after the segfault. To deal with that it is recommended to
wait few seconds after the service is down to restart it.

Error:
https://travis-ci.org/snapcore/snapd/builds/244673127#L3493

[mvo5]

👍 if it helps.

[codecov-io]

Codecov Report

Merging #3494 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3494      +/-   ##
==========================================
+ Coverage   77.18%   77.18%   +<.01%     
==========================================
  Files         375      377       +2     
  Lines       25837    25881      +44     
==========================================
+ Hits        19941    19977      +36     
- Misses       4136     4141       +5     
- Partials     1760     1763       +3

Impacted Files	Coverage Δ
overlord/hookstate/hookmgr.go	65.58% <0%> (-2.48%)	⬇️
cmd/snap/cmd_aliases.go	94% <0%> (-2%)	⬇️
overlord/overlord.go	80.24% <0%> (-0.77%)	⬇️
store/store.go	79.44% <0%> (-0.18%)	⬇️
overlord/cmdstate/cmdmgr.go	92% <0%> (ø)
overlord/cmdstate/cmdstate.go	100% <0%> (ø)
interfaces/sorting.go	96.66% <0%> (+3.33%)	⬆️
osutil/exec.go	73.83% <0%> (+3.33%)	⬆️

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