Start adding in simple wrapper around lgov/genhtml called gather_coverage

It will be installed to /usr/tests/tools for the time being

Based loosely on make snippet seen in
https://github.com/yaneurabeya/scratch/blob/master/demos/freebsd/runtime-coverage/Makefile .
The real difference is that one needs to run a binary with coverage compiled
in before running this script, so it can hoover up the .gcda's.