test_cancel: make tests a little less timing sensitive

- increase resolution of sleep/check in loop:/
- add 0.5s more tolerance to tests
diff --git a/serial/urlhandler/protocol_loop.py b/serial/urlhandler/protocol_loop.py
index 5bba4d3..7bf6cf9 100644
--- a/serial/urlhandler/protocol_loop.py
+++ b/serial/urlhandler/protocol_loop.py
@@ -189,8 +189,8 @@
             # must wait so that unit test succeeds
             time_left = self._write_timeout
             while time_left > 0 and not self._cancel_write:
-                time.sleep(min(time_left, 1))
-                time_left -= 1
+                time.sleep(min(time_left, 0.5))
+                time_left -= 0.5
             if self._cancel_write:
                 return 0  # XXX
             raise writeTimeoutError