run-tests: introduce a `HGTEST_REAL_HG` variable for test

Authored by marmoute.


run-tests: introduce a HGTEST_REAL_HG variable for test

It turns out that currently, hg and which hg can point to different things
because hg is an alias… This is annoying because script and pieces of test
are unknowingly using the wrong hg.

We will fix it in another changeset. However some test actually need to use a
real hg binary and not some chg or rhg equivalent. So we introduce a new
variable with the right value and we put it to us in the appropriate location.

Differential Revision: https://phab.mercurial-scm.org/D11049