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
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