scale number of simultaneous tests on Jenkins (#767)

Author: mr-c

jenkins.bash

@@ -58,7 +58,7 @@ do
 	fi
 	# shellcheck disable=SC2086
 	LC_ALL=C ./run_test.sh --junit-xml=result${PYTHON_VERSION}.xml RUNNER=cwltool \
-		-j4 DRAFT="${version}" "${EXTRA}" \
+		-j$(nproc) DRAFT="${version}" "${EXTRA}" \
 		"--classname=py${PYTHON_VERSION}_${CONTAINER}"
 	# LC_ALL=C is to work around junit-xml ASCII only bug
 	CODE=$((CODE+$?)) # capture return code of ./run_test.sh