tests/README

debugformat: fix formatting for compression level

`bytes(<int>)` gives a very different result as `str(<int>)` and the display
of `hg debugformat` have been broken for a while as a result.

To run the tests, do:

cd tests/
python run-tests.py

See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.