Refactoring tests

[carusogabriel]

I've refactored some tests, using:

• assertFileExists instead of assertTrue(file_exists()).

I've also used PHPUnit\Framework\TestCase instead of PHPUnit_Framework_TestCase. This will help us migrate to PHPUnit 6, that no longer support snake case class names.

I just need to bump PHPUnit version to ^4.8.35, that support this namespace.

[codecov-io]

Codecov Report

Merging #177 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #177   +/-   ##
=========================================
  Coverage     99.61%   99.61%           
  Complexity      195      195           
=========================================
  Files            22       22           
  Lines           514      514           
=========================================
  Hits            512      512           
  Misses            2        2

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[kylekatarnls]

Thanks :)