Parallelize tests across multiple CPUs (#252)

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Co-authored-by: frostedoyster <[email protected]>

tox.ini

@@ -61,12 +61,14 @@ passenv = *
 deps =
     pytest
     pytest-cov
+    pytest-xdist
 changedir = tests
 extras = soap-bpnn  # this model is used in the package tests
 allowlist_externals = bash
 commands_pre = bash {toxinidir}/tests/resources/generate-outputs.sh
 commands =
     pytest \
+        --numprocesses=auto \
         --cov={env_site_packages_dir}/metatrain \
         --cov-append \
         --cov-report= \