Cancel shutdown when crashed worker is restarted

Otherwise, a test rescheduled from pytest_handlecrashitem hook won't have a
chance to run.

Fix test failure from the previous commit.

Author: amezin

changelog/813.bugfix

@@ -0,0 +1 @@
+Cancel shutdown when a crashed worker is restarted.

src/xdist/dsession.py

@@ -225,6 +225,7 @@ def worker_errordown(self, node, error):
             self.triggershutdown()
         else:
             self.report_line("\nreplacing crashed worker %s" % node.gateway.id)
+            self.shuttingdown = False
             self._clone_node(node)
         self._active_nodes.remove(node)