Make 3D acoustics tests run faster by using smaller grid.
Make psystem_2d tests run faster by using shorter time and smaller grid.
Modify acoustics 2d variable test to be shorter

Also add specific regression data for classic and sharpclaw.  This was needed
since we now use a much coarser grid.
More description of test parameters.
Merge pull request #431 from mandli/short-tests

Short tests