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