test(ConfigFile): minor improvements

[raphinesse]

• Improves casing of module variable name (configFile -> ConfigFile)
• Get fresh module for each test to be able to make proper use of rewire

In preparation of merging #124.

[codecov-io]

Codecov Report

Merging #134 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #134   +/-   ##
=======================================
  Coverage   88.04%   88.04%           
=======================================
  Files          20       20           
  Lines        1171     1171           
=======================================
  Hits         1031     1031           
  Misses        140      140

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