Fix PIT JUnit 5 compatibility support

[Stephan202]

The migration from TestNG to JUnit 5 broke support for mutation coverage calculation. This change restores support.

See also hcoles/pitest#284.

[coveralls]

Pull Request Test Coverage Report for Build 336

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.806%

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Totals
Change from base Build 334:	0.0%
Covered Lines:	125
Relevant Lines:	144

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💛 - Coveralls

[ghost]

DeepCode's analysis on #7d7de4 found:

• :x: 0 critical issues. :warning: 0 warnings and 0 minor issues. :heavy_check_mark: 0 issues were fixed.

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[codecov[bot]]

Codecov Report

Merging #46 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #46   +/-   ##
=========================================
  Coverage     85.41%   85.41%           
  Complexity       54       54           
=========================================
  Files             5        5           
  Lines           144      144           
  Branches          1        1           
=========================================
  Hits            123      123           
  Misses           19       19           
  Partials          2        2

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[Stephan202]

Since this PR was opened most Maven configuration was migrated to oss-parent; this particular issue was fixed by PicnicSupermarket/oss-parent#6.