perf-stream-locked-section: advertise the right version key in the help As the v3 format is still experimental, its key is "v3-exp". The help text was not pointing that out. (we also fix `perf::stream-generate` in the process)

To run the tests, do:

cd tests/
python run-tests.py

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