Allow loading PHPUnit extensions via command-line options

[Jean85]

This is WIP, opening for feedback and looking at CI results.

This attempts to implement #4131. Ready for review now.

[codecov]

Codecov Report

Merging #4136 into master will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master    #4136      +/-   ##
============================================
- Coverage     83.68%   83.66%   -0.03%     
- Complexity     4484     4488       +4     
============================================
  Files           218      218              
  Lines         10994    11005      +11     
============================================
+ Hits           9200     9207       +7     
- Misses         1794     1798       +4

Impacted Files	Coverage Δ	Complexity Δ
src/TextUI/Help.php	100% <ø> (ø)	25 <0> (ø)	⬇️
src/TextUI/Arguments/Arguments.php	69% <100%> (+0.16%)	325 <82> (+1)	⬆️
src/TextUI/Arguments/ArgumentsMapper.php	80% <100%> (+0.13%)	74 <0> (ø)	⬇️
src/TextUI/Arguments/ArgumentsBuilder.php	79.64% <40%> (-0.52%)	109 <0> (+2)
src/TextUI/TestRunner.php	63.52% <50%> (-0.05%)	210 <0> (+1)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 279faa2...eadb8c8. Read the comment docs.

[Jean85]

I tested this with facile-it/paraunit#149, and it works!

That PR is a long way to be fully functional, but with this simple change it's starting to run: facile-it/paraunit@2919622#diff-6fad06c8b70bf58250bc1c7b628bdcf6R38-R47

Removing WIP, ready to review.