doc: Include documentation for test targeting via PYTEST_TESTS.

Also update the table of optional environment variables to set when
running the integration tests.

Author: s373nZ

doc/contribute-to-core-lightning/testing.md

@@ -47,6 +47,10 @@ There are four kinds of tests:
 
   You can also append `-k TESTNAME` to run a single test.  Environment variables `DEBUG_SUBD=<subdaemon>` and `TIMEOUT=<seconds>` can be useful for debugging subdaemons on individual tests, and `DEBUG_LIGHTNINGD` for attaching a debugger to each `lightningd` instance created.
 
+  Or alternatively, run a single test via the `Makefile`, by setting `PYTEST_TESTS`:
+
+  `PYTEST_TESTS="tests/test_askrene.py::test_layers" make pytest`
+
 - **pylightning tests** - will check contrib pylightning for codestyle and run the tests in `contrib/pylightning/tests` afterwards:
 
   `make check-python`