CI: replace phpdbg with php in coverage

[ohmyfelix]

Summary

• run coverage with \ instead of deprecated \
• keep the existing tester coverage target intact otherwise

Motivation

• resolves Remove phpdbg from coverage targets and use php instead contributte#73 for this repository

Testing

• make -n coverage
• full test suite

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.26%. Comparing base (db6caec) to head (353089f).
⚠️ Report is 13 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (db6caec) and HEAD (353089f). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (db6caec)	HEAD (353089f)
	6	1

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
- Coverage   52.25%   44.26%   -7.99%     
==========================================
  Files           4        4              
  Lines         111      122      +11     
==========================================
- Hits           58       54       -4     
- Misses         53       68      +15     

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