pytest: don't test crashing under valgrind at all.

Travis failures:

valgrind: m_scheduler/sema.c:104 (vgModuleLocal_sema_down): Assertion 'sema->owner_lwpid != lwpid' failed.
host stacktrace:
==1296==    at 0x38083F48: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x38084064: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x380841F1: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x38135DAE: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x380D328D: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x3809A4AC: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x3809AE43: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
==1296==    by 0x380988CF: ??? (in /usr/lib/valgrind/memcheck-amd64-linux)
sched status:
  running_tid=0
Thread 1: status = VgTs_WaitSys (lwpid 1296)
==1296==    at 0x5729730: __poll_nocancel (syscall-template.S:84)
==1296==    by 0x4348DF: daemon_poll (daemon.c:78)
==1296==    by 0x4169E7: io_poll_lightningd (lightningd.c:543)
==1296==    by 0x471ECD: io_loop (poll.c:282)
==1296==    by 0x416E06: main (lightningd.c:744)

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

tests/test_misc.py

@@ -947,8 +947,8 @@ def test_logging(node_factory):
     wait_for(check_new_log)
 
 
-@unittest.skipIf(VALGRIND and not DEVELOPER,
-                 "Backtrace upsets valgrind: only suppressed in DEVELOPER mode")
+@unittest.skipIf(VALGRIND,
+                 "Valgrind sometimes fails assert on injected SEGV")
 def test_crashlog(node_factory):
     l1 = node_factory.get_node(may_fail=True)