graceful_restart_test: Assert timer not expired
graceful_restart_test: Assert timer not expired

This patch fixes to assert the Graceful Restart time is not expired
while waiting for re-establishing peer connection.

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>

osrg/gobgp,a16/gobgp,a16/gobgp,a16/gobgp,a16/gobgp,fujita/gobgp,a16/gobgp,fujita/gobgp,tamihiro/gobgp,a16/gobgp,osrg/gobgp,tamihiro/gobgp,osrg/gobgp,tamihiro/gobgp,osrg/gobgp,tamihiro/gobgp,j-kato/gobgp,a16/gobgp,fujita/gobgp,j-kato/gobgp,fujita/gobgp,j-kato/gobgp