Hadrian fails to set MAKE when running testsuite

For a long time I have noticed, but worked around, the pecularity that I was unable to reproduce failures of some tests (e.g. often haddockHtmlTest) under Hadrian.

I finally investigated this today and have identified the culprit: make implicitly sets the MAKE environment variable in all child processes. It turns out that runtest.py came to rely on this fact. By contrast, under Hadrian MAKE is generally unset, meaning that a command line like $MAKE --no-print-directory htmlTest will fail. In the case of haddockHtmlTest we don't print stderr, hence the resulting error message is hidden.