run-tests: allow different extra weight for slow tests
The 'test-check-code-hg.t' file is not big enough to be prioritized properly.
As a result my tests run often spend about 15 seconds running only it at the
end of its tests run. We make the "slow" mechanism a bit smarter to adjust the
extra weight of each category independently in a future patch.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.