Bump number of retries in missing_xlog test.

Failures are seen in CI randomly where
checkpoint_and_wait_for_replication_replay return false, but when check on the
box later the primaries and mirrors are in sync. So, seems its timing related
failures maybe sometimes containers are running slow causing delays. This test
is for checking functionality and not performance, so avoiding flaky failures is
better with increased number of retries, which in general shouldn't affect
runtime.

src/test/walrep/expected/missing_xlog.out

@@ -105,7 +105,7 @@ begin
 end;
 $$ language plpgsql;
 -- checkpoint to ensure clean xlog replication before bring down mirror
-select checkpoint_and_wait_for_replication_replay(200);
+select checkpoint_and_wait_for_replication_replay(500);
  checkpoint_and_wait_for_replication_replay 
 --------------------------------------------
  t
@@ -171,7 +171,7 @@ select pg_ctl((select datadir from gp_segment_configuration c where c.role='m' a
 (1 row)
 
 -- check the view, we expect to see error
-select wait_for_replication_error('walread', 0, 200);
+select wait_for_replication_error('walread', 0, 500);
  wait_for_replication_error 
 ----------------------------
  t
@@ -191,7 +191,7 @@ select move_xlog('/tmp/missing_xlog', (select datadir || '/pg_xlog' from gp_segm
 (1 row)
 
 -- the error should go away
-select wait_for_replication_error('none', 0, 200);
+select wait_for_replication_error('none', 0, 500);
  wait_for_replication_error 
 ----------------------------
  t

src/test/walrep/sql/missing_xlog.sql

@@ -110,7 +110,7 @@ end;
 $$ language plpgsql;
 
 -- checkpoint to ensure clean xlog replication before bring down mirror
-select checkpoint_and_wait_for_replication_replay(200);
+select checkpoint_and_wait_for_replication_replay(500);
 
 create extension if not exists gp_inject_fault;
 -- Prevent FTS from probing segments as we don't want a change in
@@ -137,14 +137,14 @@ select move_xlog((select datadir || '/pg_xlog' from gp_segment_configuration c w
 select pg_ctl((select datadir from gp_segment_configuration c where c.role='m' and c.content=0), 'start', (select port from gp_segment_configuration where content = 0 and preferred_role = 'm'), 0);
 
 -- check the view, we expect to see error
-select wait_for_replication_error('walread', 0, 200);
+select wait_for_replication_error('walread', 0, 500);
 select sync_error from gp_stat_replication where gp_segment_id = 0;
 
 -- bring the missing xlog back on segment 0
 select move_xlog('/tmp/missing_xlog', (select datadir || '/pg_xlog' from gp_segment_configuration c where c.role='p' and c.content=0));
 
 -- the error should go away
-select wait_for_replication_error('none', 0, 200);
+select wait_for_replication_error('none', 0, 500);
 select sync_error from gp_stat_replication where gp_segment_id = 0;
 
 -- Resume FTS probes and perform a probe scan.