Forbid custom PHPUnit binary as ParaTest is coupled with pinned PHPUnit version

[Slamdunk]

No description provided.

[codecov]

Codecov Report

Merging #528 into master will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master      #528   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       546       546           
===========================================
  Files             24        24           
  Lines           1668      1658   -10     
===========================================
- Hits            1668      1658   -10     

Impacted Files	Coverage Δ	Complexity Δ
src/Runners/PHPUnit/Options.php	100.00% <100.00%> (ø)	96.00 <0.00> (ø)

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