tests: remove the custom run-tests.py This test-runners is a copy from the Mercurial one and slowly drift out of sync. Use the Mercurial's core test runners instead.

make: clean test related target We removed the runner, so they are broken. We can reintroduce them later with proper data to fetch an official 'run-tests.py' runner.

test: use 'dummyssh' from the run-tests directory This is a step toward not shipping our own test runners and relying on the official Mercurial one.