pytest: Use the pytest-timeout plugin to kill tasks

This will kill a test that was running for 550 seconds, so that we
get a traceback before the travis inactivity timeout of 600 seconds
kicks in. The traceback should show us which test got stuck and where
it got stuck.

Signed-off-by: Christian Decker <@cdecker>

Makefile

@@ -38,7 +38,9 @@ ifeq ($(COMPAT),1)
 COMPAT_CFLAGS=-DCOMPAT_V052=1 -DCOMPAT_V060=1 -DCOMPAT_V061=1
 endif
 
-PYTEST_OPTS := -v
+# Timeout shortly before the 600 second travis silence timeout
+# (method=thread to support xdist)
+PYTEST_OPTS := -v --timeout=550 --timeout_method=thread
 
 # This is where we add new features as bitcoin adds them.
 FEATURES :=