Add returncode argument to pytest.exit

If the argument is not None, it'll raise a SystemExit exception to
cleanly exit pytest.

src/_pytest/outcomes.py

@@ -57,9 +57,12 @@ class Exit(KeyboardInterrupt):
 # exposed helper methods
 
 
-def exit(msg):
+def exit(msg, returncode=None):
     """ exit testing process as if KeyboardInterrupt was triggered. """
     __tracebackhide__ = True
+    if returncode:
+        raise SystemExit(returncode)
+    else:
         raise Exit(msg)