The h5diff program returns 1 (failure) if there are any differences at all,
even if using a relative or absolute difference tolerance and seeing no
differences above the tolerance. In that situation, the output will be empty,
but the return value 1.
To make this test behave similar to the other regression tests, we now check
if its output is empty instead.
Also: set relative tolerance to 0.01 and remove the absolute test. This test
is not meant to trigger for numerics changes, but should capture errors in
the writing of the HDF5 file.
