Added the checkpoint call in the snapfs_get_snapshot_size test.

Author: rzharkov

src/test/isolation/expected/snapfs_get_snapshot_size.out

@@ -5,6 +5,7 @@ step s1_mk_sn:
 	select 1 as "make_snapshot" from pg_make_snapshot();
 	select pg_get_snapshot_size( ( select recent_snapshot from pg_control_snapshot() ) );
 	create table foo as select 'test_data';
+	checkpoint;
 	select pg_get_snapshot_size( ( select recent_snapshot from pg_control_snapshot() ) ) > 0 as size_not_zero;
 
 make_snapshot  

src/test/isolation/specs/snapfs_get_snapshot_size.spec

@@ -8,6 +8,7 @@ step "s1_mk_sn" {
 	select 1 as "make_snapshot" from pg_make_snapshot();
 	select pg_get_snapshot_size( ( select recent_snapshot from pg_control_snapshot() ) );
 	create table foo as select 'test_data';
+	checkpoint;
 	select pg_get_snapshot_size( ( select recent_snapshot from pg_control_snapshot() ) ) > 0 as size_not_zero;
 }