Generate mocha JSON output with --matrix

[cgewecke]

Writes data collected by Mocha's JSON reporter to mochaOutput.json when using the --matrix flag.

tarantula consumes both the test matrix and the (newly added) pass/fail report.

[codecov-io]

Codecov Report

Merging #601 (ded8006) into experimental-options (308f32d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@                  Coverage Diff                  @@
##           experimental-options     #601   +/-   ##
=====================================================
  Coverage                 99.17%   99.17%           
=====================================================
  Files                        23       23           
  Lines                      1214     1217    +3     
=====================================================
+ Hits                       1204     1207    +3     
  Misses                       10       10           

Impacted Files	Coverage Δ
plugins/resources/nomiclabs.utils.js	98.87% <ø> (ø)
plugins/resources/truffle.utils.js	100.00% <ø> (ø)
lib/api.js	100.00% <100.00%> (ø)

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