Make test more specific to prevent flakiness

[njsmith]

Sometimes ResourceWarnings decide to print during this test, and that
used to cause it to fail every once in a while.

[njsmith]

I was hoping #1565 would fix this, but then: https://github.com/python-trio/trio/pull/1568/checks?check_run_id=729466074

So we should probably fix it correctly...

[codecov]

Codecov Report

Merging #1569 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1569      +/-   ##
==========================================
+ Coverage   99.68%   99.69%   +0.01%     
==========================================
  Files         110      111       +1     
  Lines       13498    13866     +368     
  Branches     1027     1059      +32     
==========================================
+ Hits        13455    13824     +369     
+ Misses         28       27       -1     
  Partials       15       15              

Impacted Files	Coverage Δ
trio/tests/test_threads.py	100.00% <100.00%> (ø)
trio/lowlevel.py	100.00% <0.00%> (ø)
trio/_core/__init__.py	100.00% <0.00%> (ø)
trio/_core/_io_epoll.py	100.00% <0.00%> (ø)
trio/_core/_wakeup_socketpair.py	100.00% <0.00%> (ø)
trio/_core/tests/test_guest_mode.py	100.00% <0.00%> (ø)
trio/_core/_run.py	99.76% <0.00%> (+0.02%)	⬆️
trio/_core/_io_windows.py	98.57% <0.00%> (+0.03%)	⬆️
trio/_core/_ki.py	100.00% <0.00%> (+1.49%)	⬆️
trio/_core/_io_kqueue.py	85.00% <0.00%> (+1.98%)	⬆️