Stop camel-casing testfiles flag

[cgewecke]

Buidler requires command line params to be lowercase

Error BDLR310: Invalid param --testFiles. Command line params must be lowercase.

[codecov-io]

Codecov Report

Merging #443 into buidler-plugin will decrease coverage by 30.55%.
The diff coverage is 100%.

@@                 Coverage Diff                 @@
##           buidler-plugin     #443       +/-   ##
===================================================
- Coverage           99.21%   68.66%   -30.56%     
===================================================
  Files                  18       17        -1     
  Lines                 768      766        -2     
===================================================
- Hits                  762      526      -236     
- Misses                  6      240      +234

Impacted Files	Coverage Δ
dist/buidler.plugin.js	96.55% <100%> (-3.45%)	⬇️
dist/plugin-assets/truffle.utils.js	10.6% <0%> (-89.4%)	⬇️
dist/truffle.plugin.js	16.07% <0%> (-83.93%)	⬇️
lib/preprocessor.js	46.15% <0%> (-50.01%)	⬇️
lib/registrar.js	62.5% <0%> (-37.5%)	⬇️
lib/injector.js	68.62% <0%> (-31.38%)	⬇️
dist/plugin-assets/truffle.ui.js	69.23% <0%> (-30.77%)	⬇️
dist/plugin-assets/buidler.ui.js	71.42% <0%> (-28.58%)	⬇️
lib/parse.js	67.6% <0%> (-28.17%)	⬇️
lib/collector.js	76.47% <0%> (-23.53%)	⬇️
... and 9 more

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