Enable parallel testing in CI

[jbergstroem]

A bunch of people has helped out improving running the test suite in parallel
(refs: nodejs/node#4476). What left to be done now is to roll it out in CI. The tricky part here is that passing -j $(prtconf stuff) is that its not cross platform and is plain wrong on some systems (smartos). Passing -J to test.py won't work either since it uses pythons multiprocessing.cpu_count() which similar to /proc/cpu will be way off on some os'es (hi, smartos).

My idea was that we could start rolling out the environment variable JOBS= which would reflect the amount of parallel threads we want a slave to use. If this is found in Makefile (or vcbuild), we pass it to -j $JOBS; otherwise we move on as usual (no -j).

Thoughts? Downsides?

Edit: elaborating on -J in python.

[jbergstroem]

The makefile change would look something like this (in nodejs):

diff --git Makefile Makefile
index 401464c..7e7395b 100644
--- Makefile
+++ Makefile
@@ -167,8 +167,13 @@ test-all: test-build test/gc/node_modules/weak/build/Release/weakref.node
 test-all-valgrind: test-build
        $(PYTHON) tools/test.py --mode=debug,release --valgrind

+
+ifdef JOBS
+TEST_OPTS=-j $(JOBS)
+endif
 test-ci: | build-addons
-       $(PYTHON) tools/test.py -p tap --logfile test.tap --mode=release --flaky-tests=$(FLAKY_TESTS) \
+       $(PYTHON) tools/test.py $(TEST_OPTS) -p tap --logfile test.tap \
+               --mode=release --flaky-tests=$(FLAKY_TESTS) \
                $(TEST_CI_ARGS) addons message parallel sequential

 test-release: test-build

..and we'll do something like JOBS={{ ansible_processor_cores }} or JOBS=$(getconf _NPROCESSORS_ONLN) in init scripts for architectures that detects it properly. For the other ones we'll override.

[jbergstroem]

Landed in node a few days ago: nodejs/node@f49a1d0. The next step is to add JOBS to all build environments.