`test-pull-many.sh` is was just too slow to be a unit test. Generating
a bunch of files via shell is slow, the delta generation is slow, etc.
Every developer doesn't need to run it every time.
Somewhat address this by converting it into our installed test framework, which
moves it out of the developer fast paths. Another advantage to this is
that we can simply reuse the FAH tree content rather than synthesizing
new bits each time.
Closes: #840
Approved by: jlebon
Colin Walters