Fix a test that was assuming to be run on UTC clock.

[pcorpet]

Fix #604

[pcorpet]

@felixonmars, can you take a look please?

[felixonmars]

@pcorpet Thanks! I confirm that it fixes the error on my UTC+8 machine.

[codecov]

Codecov Report

Merging #652 into develop will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #652      +/-   ##
===========================================
+ Coverage    95.16%   95.19%   +0.02%     
===========================================
  Files           17       17              
  Lines         3354     3354              
===========================================
+ Hits          3192     3193       +1     
+ Misses         162      161       -1     

Impacted Files	Coverage Δ
mongomock/filtering.py	98.95% <0.00%> (+0.34%)	⬆️

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