Use the env keyword of `test_compare.py`

Use the env keyword of `test_compare.py` rather than setting env on
the command line. This seems a little clearer and it allows us to
remove the stdout line in the test description. The possibility to add
env was added to `test_compare.py` in commit 4964d767 of the
`Test_compare` repository, after replacing mpiexec by `$mpiexec` in
`eddy_graph.sh`, in commit 850efa4b of the present repository.