CI: Run tests with available parallelism

Author: effigies

setup.cfg

@@ -24,6 +24,7 @@ test =
     pytest-cov
     pytest-doctestplus
     pytest-httpserver
+    pytest-xdist
 zstd =
     pyzstd >= 0.14.3
 all =

tools/ci/check.sh

@@ -25,7 +25,7 @@ elif [ "${CHECK_TYPE}" == "test" ]; then
     cd for_testing
     cp ../.coveragerc .
     pytest --doctest-modules --doctest-plus --cov nibabel --cov-report xml \
-        --junitxml=test-results.xml -v --pyargs nibabel
+        --junitxml=test-results.xml -v --pyargs nibabel -n auto
 else
     false
 fi