Improved checks for (in)valid gmxtest.pl command lines

* usage message should be printed when there was no work,
  and not when there was legitimately no work
* "./gmxtest.pl -tight" now triggers the usage message,
  because the (wrong) dependence on $kk is removed
* whether or not cleaning took place is tested explicitly,
  rather than implicitly in a way that might be triggered
  erroneously by some other code path (i.e. empty @work).
  Adding cleaning to @work triggers the checks whether
  GROMACS executables are found, etc., so that is not
  the right solution.

Change-Id: Ifc6fe6bc807cb74b23eaf45c1dbfe03c45fc37da