Run regressiontests from build

Run regressiontests from build

If either REGRESSIONTEST_PATH or REGRESSIONTEST_DOWNLOAD is set,
the regressiontests are run as ctests. The later option auto-
downloads the regressiontest repo.

The "check" target builds and runs tests in one step.
The "gmxtests" target builds tests.
If neither variable is set by the user, the commit doesn't change
the prior behavior (no messsages). Running the new target "check"
without either variable set, prints a warnings.

This commit also removes the prior pseudo test of running
"mdrun -h".

Change-Id: I103648f8a87891727a70775b19449fc269002a1c