Two small changes to tests

I think the dependency on ENABLE_IMDIAG should be dropped, as configure makes sure it's enabled if testbench is. Additionally, it seems that the set of tests that requires imdiag is bigger that what was specified in the make file. Therefore, rather than correcting it, I just got rid of it.

@Rainer: not much of a cleanup so far, I just wanted to push this early as you also seem to be making changes in /tests. I plan to look at make distcheck next.