improved testing section

I went to the Nakamoto dinner last week and told some guys they
could get involved by improving our test coverage. So I updated
the docs for newbs like me. (I only recently discovered `PYTEST_PAR`).

Signed-off-by: Mark Beckwith <wythe@intrig.com>

doc/HACKING.md

@@ -90,7 +90,7 @@ Here's a list of parts, with notes:
 
 * closingd/ - daemon to handle mutual closing negotiation with a single peer.
 
-* onchaind/ - daemon to hand a single channel which has had its funding
+* onchaind/ - daemon to handle a single channel which has had its funding
   transaction spent.
 
 Debugging
@@ -160,27 +160,52 @@ lightning funds.
 
 Testing
 -------
+Install `valgrind` and `pytest-xdist` for best results:
 
-There are three kinds of tests.  For best results, you should have
-valgrind installed, and build with DEVELOPER=1.
+```
+sudo apt install valgrind
+pip3 install pytest-xdist
+```
 
-* source tests - run by `make check-source`, looks for whitespace,
+Tests are run with: `make check [flags]` where the pertinent flags are:
+
+```
+DEVELOPER=[0|1] - developer mode increases test coverage
+VALGRIND=[0|1]  - detects memory leaks during test execution but adds a significant delay
+PYTEST_PAR=n    - runs pytests in parallel
+```
+
+A modern desktop can build and run through all the tests in a couple of minutes with:
+
+    make -j12 check PYTEST_PAR=24 DEVELOPER=1 VALGRIND=0
+
+Adust `-j` and `PYTEST_PAR` accordingly for your hardware.
+
+There are three kinds of tests:
+
+* **source tests** - run by `make check-source`, looks for whitespace,
   header order, and checks formatted quotes from BOLTs if BOLTDIR
   exists (currently disabled, since BOLTs are being re-edited).
 
-* unit tests - run by `make check`, these are `run-*.c` files in test/
-  subdirectories which can test routines inside C source files.
-  You should insert `/* AUTOGENERATED MOCKS START */` and
-  `/* AUTOGENERATED MOCKS END */` lines, and `make update-mocks`
-  will automatically generate stub functions which will allow you to
-  link (which will conveniently crash if they're called).
+* **unit tests** - standalone programs that can be run individually.
+  They are `run-*.c` files in test/ subdirectories used to test routines
+  inside C source files.
+
+  You should insert the lines when implementing a unit test:
+
+  `/* AUTOGENERATED MOCKS START */`
+
+  `/* AUTOGENERATED MOCKS END */`
+
+  and `make update-mocks` will automatically generate stub functions which will
+  allow you to link (and conveniently crash if they're called).
+
+* **blackbox tests** - These test setup a mini-regtest environment and test
+  lightningd as a whole.  They can be run individually:
 
-* blackbox tests - run by `make check` or directly as
   `PYTHONPATH=contrib/pylightning python3 tests/test_lightningd.py -f`.
-  You can run these much faster by putting `VALGRIND=0` after `make check`,
-  which has the added bonus of doing
-  memory leak detection.  You can also append `LightningDTests.TESTNAME`
-  to run a single test.
+
+  You can also append `LightningDTests.TESTNAME` to run a single test.
 
 Our Travis CI instance (see `.travis.yml`) runs all these for each
 pull request.