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
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