overlord: rely on more conservative ensure interval

[zyga]

On openSUSE tumbleweed running Linux 4.11.8 and golang 1.8.3 the
observed latency between subsequent calls to Ensure in
overlordSuite.EnsureLoopPrune is on the order of ~500ms versus ~50ms on
of the same binary running on top of Ubuntu Artful with Linux 4.11.0 and
golang 1.8.1. This makes the test fail as the sequence of evens it
widely different and things just don't happen in time.

As a workaround, before the root issue is understood better, use a more
conservative prune interval of 200ms (up from 20ms) which makes this test
pass each time.

Signed-off-by: Zygmunt Krynicki me@zygoon.pl

[codecov-io]

Codecov Report

Merging #3716 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3716      +/-   ##
==========================================
- Coverage   75.65%   75.65%   -0.01%     
==========================================
  Files         391      391              
  Lines       33849    33849              
==========================================
- Hits        25609    25607       -2     
- Misses       6418     6420       +2     
  Partials     1822     1822

Impacted Files	Coverage Δ
wrappers/binaries.go	68.96% <0%> (-10.35%)	⬇️
cmd/snap/cmd_aliases.go	93.33% <0%> (-1.67%)	⬇️
interfaces/sorting.go	98.71% <0%> (-1.29%)	⬇️
overlord/ifacestate/helpers.go	62.33% <0%> (ø)	⬆️
overlord/snapstate/snapstate.go	80.72% <0%> (+0.26%)	⬆️

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