Move gcovr options to config file

[iluuu1994]

Having these in the make file is very annoying, because it requires rebuilding from scratch and rerunning tests just to see if some flag is effective.

Note: gcovr config files are marked as experiemental:

https://gcovr.com/en/latest/guide/configuration.html

However, they've been experimental for ~4 years and it's unlikely it'll go anywhere. Even if, we use this for internal purposes only.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.77%. Comparing base (6d62d87) to head (13621a7).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21663   +/-   ##
=======================================
  Coverage   70.77%   70.77%           
=======================================
  Files         812      812           
  Lines      296976   296976           
  Branches    84534    84534           
=======================================
  Hits       210199   210199           
+ Misses      56610    56608    -2     
- Partials    30167    30169    +2     

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