pytest: test that we unreserve txprepare inputs across shutdown/crash.

We fail this at the moment, since we rely on shutdown to do the cleanups
for us.

(Also had to fix the unclean shutdown path: the caller checks the rc unless
 mayfail is set, and of course it's not zero since we just SIGTERM'd it).

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

tests/utils.py

@@ -149,9 +149,6 @@ class TailableProc(object):
         self.proc.wait()
         self.thread.join()
 
-        if self.proc.returncode:
-            raise ValueError("Process '{}' did not cleanly shutdown: return code {}".format(self.proc.pid, rc))
-
         return self.proc.returncode
 
     def kill(self):