run-tests: rely on an actual executable in PATH instead of alias for `hg` The alias approach is poorly inherited by other process that the test might spawn. To solve this we use the same approach as for `python`/`python3` we write an executable file explicitly. Doing this fixes `which hg` invocation that now returns the same location as `hg`. Using chg server side has some minor effect on some stdout/stderr ordering when using `chg` as the server too. Differential Revision: https://phab.mercurial-scm.org/D11053