Automatic test coverage reports

[gavv]

Last revised: Oct 2023

It would be nice to have test coverage reports.

Steps:

• [ ] Add a new scons option --enable-coverage (to SConstruct) to enable test coverage on GCC and Clang.
• [ ] Add a new scons target coverage (or cov or something) that generates local HTML report using lcov tool.
• [ ] In CI builds, collect coverage reports and publish them to https://coveralls.io using cpp-coveralls (or we can use another service and tools if you know a better alternative).

Example of this approach: https://github.com/google/benchmark/pull/122