bcachefs code coverage analysis

[Kent Overstreet <kent.overstreet@linux.dev>]

Just got automated gcov analysis going - integrated with the CI. Thanks
Peter for supplying a missing bit of make magic :)

I'll be writing a longer post about ktest/ktestci soon... been doing a
lot of bugfixing and scalability work, it's humming along nicely now.

For now I just wanted to show off the coverage results. ktest now has
gcov variants of our existing tests, we only run these on the most
recent commit of the master branch (vs. the most recent 50 commits for
the other tests), and commits that have gcov results now give you a nice
link to the lcov output, like so:

https://evilpiepirate.org/~testdashboard/ci?branch=bcachefs&commit=26ed392c9add057a503077ad87492d1ab1475407&test=^gcov
https://evilpiepirate.org/~testdashboard/c/26ed392c9add057a503077ad87492d1ab1475407/lcov/fs/bcachefs/index.html

82% line coverage is not too shabby :)

Next up we need to get new-and-improved dynamic fault injection merged:
the code is written, I'm just waiting on memory allocation profiling to
be merged to post it (which has the necessary infrastructure). That will
make it dead easy to add fault injection points for code that isn't
being tested and hook them up to new tests.