tests/README.md instructions are out of date and unclear

[moreati]

I also failed at the first step of trying to look at adding tests as the instructions here https://github.com/mitogen-hq/mitogen/blob/master/tests/README.md#steps-to-prepare-development-environment reference some scripts that don't seem to exist any more.

Originally posted by @jrosser in https://github.com/mitogen-hq/mitogen/issues/1087#issuecomment-2283689763