Add regression test for recovery pause.

Add regression test for recovery pause.

Previously there was no regression test for recovery pause feature.
This commit adds the test that checks

- recovery can be paused or resumed expectedly
- pg_get_wal_replay_pause_state() reports the correct pause state
- the paused state ends and promotion continues if a promotion
   is triggered while recovery is paused

Suggested-by: Michael Paquier
Author: Fujii Masao
Reviewed-by: Kyotaro Horiguchi, Dilip Kumar
Discussion: http://postgr.es/m/YKNirzqM1HYyk5h4@paquier.xyz