Automatically use the proper number of cores (-j was unlimited)

test/Makefile

 DISABLED_TESTS=common/
 TEST_CASES=$(filter-out $(DISABLED_TESTS), $(sort $(dir $(wildcard */))))
+export MAKEFLAGS="-j $(grep -c ^processor /proc/cpuinfo)"
 
 all:
 	@python3 test.py all $(TEST_CASES)