Use spawn not exec to run commands

[nick-fields]

As a possible fix for #84 and #76, use spawn rather than exec to run commands after @tstackhouse found that exec wasn't great at handling large outputs due to its use of buffering. Spawn is much more efficient because it streams output.

Tested this locally with 1000MiB of output using @LukeShu very helpful repro script and after a very long wait, it exited as expected. Added a test of 100MiB to CI to ensure this continues working.

[codecov-commenter]

Codecov Report

Merging #88 (b4ef326) into master (a25f198) will not change coverage.
The diff coverage is 0.00%.

@@          Coverage Diff           @@
##           master     #88   +/-   ##
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  Coverage    1.38%   1.38%           
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  Files           2       2           
  Lines         144     144           
  Branches       48      48           
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  Hits            2       2           
  Misses        131     131           
  Partials       11      11