Use a random port

[meejah]

(Pulled out of tahoe-invite PR). This at least uses a random port instead of 1234; better would be if we could not listen at all in these tests.

[codecov-io]

Codecov Report

Merging #435 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #435      +/-   ##
==========================================
- Coverage   88.02%   88.01%   -0.01%     
==========================================
  Files         147      147              
  Lines       27835    27835              
  Branches     3973     3973              
==========================================
- Hits        24501    24499       -2     
- Misses       2619     2621       +2     
  Partials      715      715

Impacted Files	Coverage Δ
src/allmydata/util/dictutil.py	97.19% <0%> (-0.71%)	⬇️
src/allmydata/web/status.py	82.86% <0%> (+0.11%)	⬆️

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[warner]

The coverage problem is unrelated.. we need to improve the dictutil tests.

We should probably change these tests so they don't actually have to start up a Tub (maybe mock it out), so the network port shouldn't matter, but this is a good interim fix.

+1, I'll land soon