tests/thirdparty: add crypto/internal/bigmod

[mmcloughlin]

Updates #354

[codecov-commenter]

Codecov Report

Merging #355 (d0d034c) into master (67039b7) will increase coverage by 0.40%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master     #355      +/-   ##
==========================================
+ Coverage   75.58%   75.98%   +0.40%     
==========================================
  Files          65       65              
  Lines       20694    20710      +16     
==========================================
+ Hits        15641    15737      +96     
+ Misses       4975     4890      -85     
- Partials       78       83       +5     

Flag	Coverage Δ
integration	11.92% <ø> (ø)
unittests	73.07% <81.81%> (+0.40%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tests/thirdparty/config.go	81.30% <81.81%> (+6.07%)	⬆️
examples/md5x16/md5x16.go	86.41% <0.00%> (+86.41%)	⬆️

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