Make dtm_recovery_on_standby test more deterministic

The test should wait for the transactions to be in the right state
before promoting standby.  This commit adds a wait step to ensure just
that.  One of the ICW jobs in CI failed because the test promoted the
standby before the transactions were preprared on master.  This should
no longer happen now.

src/test/isolation2/expected/segwalrep/dtm_recovery_on_standby.out

@@ -44,6 +44,18 @@ select gp_inject_fault('transaction_abort_failure', 'suspend', dbid) from gp_seg
 
 2&: create table aborted_by_standby(a int, b int);  <waiting ...>
 
+-- Wait for the two transactions to be suspended on master
+select gp_wait_until_triggered_fault('dtm_broadcast_commit_prepared', 1, dbid) from gp_segment_configuration where content = -1 and role = 'p';
+ gp_wait_until_triggered_fault 
+-------------------------------
+ t                             
+(1 row)
+select gp_wait_until_triggered_fault('transaction_abort_failure', 1, dbid) from gp_segment_configuration where content = -1 and role = 'p';
+ gp_wait_until_triggered_fault 
+-------------------------------
+ t                             
+(1 row)
+
 -- Promote standby
 select pg_ctl(datadir, 'promote') from gp_segment_configuration where content = -1 and role = 'm';
  pg_ctl            

src/test/isolation2/sql/segwalrep/dtm_recovery_on_standby.sql

@@ -29,6 +29,12 @@ from gp_segment_configuration where content = -1 and role = 'p';
 
 2&: create table aborted_by_standby(a int, b int);
 
+-- Wait for the two transactions to be suspended on master
+select gp_wait_until_triggered_fault('dtm_broadcast_commit_prepared', 1, dbid)
+from gp_segment_configuration where content = -1 and role = 'p';
+select gp_wait_until_triggered_fault('transaction_abort_failure', 1, dbid)
+from gp_segment_configuration where content = -1 and role = 'p';
+
 -- Promote standby
 select pg_ctl(datadir, 'promote') from gp_segment_configuration
 where content = -1 and role = 'm';