List calculators properly when running tests
When running the test suite, show a listing of installed/enabled calculators in the pytest header such as
Calculators
===========
[ ] abinit not installed
[ ] asap /home/askhl/.local/lib/python3.8/site-packages/asap3
[ ] cp2k executable=cp2k_shell
[ ] dftb executable=dftb+
[x] espresso executable=pw.x, pseudo_dir=/home/askhl/src/ase-datafiles/asetest/datafiles/espresso/gbrv-lda-espresso
[ ] gpaw /home/askhl/src/gpaw/gpaw
[ ] lammpsrun executable=lmp
[ ] octopus executable=octopus
[x] siesta executable=siesta, pseudo_path=/home/askhl/src/ase-datafiles/asetest/datafiles/siesta
Also quit with an appropriate message if user requests a calculator which is not installed.