WIP and proof of concept

A report is generated in builddir/coverage/profiles similar to
http://lab.llvm.org:8080/coverage/coverage-reports/index.html

Limitations:
Multiple runs cannot be stored yet. How I did it in my hack F9562509 was
putting timestamp of running ctest in the folder name. But it's
difficult to do it with GTests. Looking into it.

Need to find ways to generate compact report like F9507919 instead of
the expanded line by line information.

Takes a long time between finishing the test run and quitting Blender
after finishing writing profraw file. That is despite using
llvm-symbolizer.

Look into why FIXTURES_CLEANUP is not taking effect: cov_processing
should be run despite using -R blah.

Other platforms.