Read pexpect buffers befor stopping

pexpect.wait blocks forever if there is still output to be read when
it is called. try reading the buffers before calling wait to avoid the issue.

pygatt/backends/gatttool/gatttool.py

@@ -397,7 +397,13 @@ def kill(self):
         #    $ sudo setcap 'cap_net_raw,cap_net_admin+eip' `which hcitool`
         try:
             self._scan.kill(signal.SIGINT)
-            self._scan.wait()
+            while True:
+                try:
+                    self._scan.read_nonblocking()
+                except Exception:
+                    break
+            if self._scan.isalive():
+                self._scan.wait()
         except OSError:
             log.error("Unable to gracefully stop the scan - "
                       "BLE adapter may need to be reset.")