ci: generate single code coverage report for C++ code

This merge request replaces all the coverage* CTest C++ tests with a single CMake target that generates a single report for all the C++ tests instead of one report for each coverage* CTest C++ test.

In addition, the CMake target uses fastcov instead of lcov because fastcov is at least an order of magnitude faster than lcov at the moment.

Note that fastcov and lcov-to-cobertura-xml were downloaded using the following commands:

$ bash 3d_party/fastcov/download.sh
Installing from commit https://github.com/RPGillespie6/fastcov/commit/40dffe81d62c0d897afe4108f3b5489487ff3bce into 3d_party/fastcov
  - https://raw.githubusercontent.com/RPGillespie6/fastcov/40dffe81d62c0d897afe4108f3b5489487ff3bce/fastcov.py  --> fastcov.py
  - https://raw.githubusercontent.com/RPGillespie6/fastcov/40dffe81d62c0d897afe4108f3b5489487ff3bce/LICENSE     --> LICENSE
  - https://raw.githubusercontent.com/RPGillespie6/fastcov/40dffe81d62c0d897afe4108f3b5489487ff3bce/README.md   --> README.md

$ bash 3d_party/lcov-to-cobertura-xml/download.sh
Installing from commit https://github.com/eriwen/lcov-to-cobertura-xml/commit/18489f195e5389fca3fec53608a5503af759ee44 into 3d_party/lcov-to-cobertura-xml
  - https://raw.githubusercontent.com/eriwen/lcov-to-cobertura-xml/18489f195e5389fca3fec53608a5503af759ee44/LICENSE     --> LICENSE
  - https://raw.githubusercontent.com/eriwen/lcov-to-cobertura-xml/18489f195e5389fca3fec53608a5503af759ee44/README.md   --> README.md
  - https://raw.githubusercontent.com/eriwen/lcov-to-cobertura-xml/18489f195e5389fca3fec53608a5503af759ee44/lcov_cobertura/lcov_cobertura.py    --> lcov_cobertura/lcov_cobertura.py