Makefile: set PYTHONPATH to be absolute when running tests.

My test machine started failing on dynamic plugin tests, unable to
find the lightning module:

  ModuleNotFoundError: No module named 'lightning'

This is because plugins at startup are run from whatever directory
you're in, but on refresh are run from the lightning-dir.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Makefile

@@ -259,7 +259,7 @@ ifeq ($(PYTEST),)
 	exit 1
 else
 # Explicitly hand DEVELOPER and VALGRIND so you can override on make cmd line.
-	PYTHONPATH=contrib/pylightning:$$PYTHONPATH TEST_DEBUG=1 DEVELOPER=$(DEVELOPER) VALGRIND=$(VALGRIND) $(PYTEST) tests/ $(PYTEST_OPTS)
+	PYTHONPATH=`pwd`/contrib/pylightning:$$PYTHONPATH TEST_DEBUG=1 DEVELOPER=$(DEVELOPER) VALGRIND=$(VALGRIND) $(PYTEST) tests/ $(PYTEST_OPTS)
 endif
 
 # Keep includes in alpha order.