Fix flakyness in failure_create_distributed_table_non_empty (#6217)

The failure_create_distributed_table_non_empty test would sometimes fail
like this:
```diff
 -- in the first test, cancel the first connection we sent from the coordinator
 SELECT citus.mitmproxy('conn.cancel(' ||  pg_backend_pid() || ')');
- mitmproxy
----------------------------------------------------------------------
-
-(1 row)
-
+ERROR:  canceling statement due to user request
+CONTEXT:  COPY mitmproxy_result, line 1: ""
+SQL statement "COPY mitmproxy_result FROM '/home/circleci/project/src/test/regress/tmp_check/mitmproxy.fifo'"
+PL/pgSQL function citus.mitmproxy(text) line 11 at EXECUTE
 SELECT create_distributed_table('test_table', 'id');
```

Because the cancel command had no filter it would actually sometimes
cancel the mitmproxy cancel command itself. This PR addresses that by
filtering on CREATE TABLE, which is one of the command that
create_distributed_table will send to the workers.

Example of failing test: https://app.circleci.com/pipelines/github/citusdata/citus/26252/workflows/1b7e5464-cca4-4ec1-99b3-48ddf25c29fa/jobs/742829

(cherry picked from commit e2a24b921e)

src/test/regress/expected/failure_create_distributed_table_non_empty.out

@@ -614,7 +614,7 @@ SELECT run_command_on_workers($$SELECT count(*) FROM information_schema.tables W
 (2 rows)
 
 -- in the first test, cancel the first connection we sent from the coordinator
-SELECT citus.mitmproxy('conn.cancel(' ||  pg_backend_pid() || ')');
+SELECT citus.mitmproxy('conn.onQuery(query="CREATE TABLE").cancel(' ||  pg_backend_pid() || ')');
  mitmproxy
 ---------------------------------------------------------------------
 

src/test/regress/sql/failure_create_distributed_table_non_empty.sql

@@ -224,7 +224,7 @@ SELECT count(*) FROM pg_dist_shard WHERE logicalrelid='create_distributed_table_
 SELECT run_command_on_workers($$SELECT count(*) FROM information_schema.tables WHERE table_schema = 'create_distributed_table_non_empty_failure' and table_name LIKE 'test_table%'$$);
 
 -- in the first test, cancel the first connection we sent from the coordinator
-SELECT citus.mitmproxy('conn.cancel(' ||  pg_backend_pid() || ')');
+SELECT citus.mitmproxy('conn.onQuery(query="CREATE TABLE").cancel(' ||  pg_backend_pid() || ')');
 SELECT create_distributed_table('test_table', 'id');
 SELECT citus.mitmproxy('conn.allow()');
 SELECT count(*) FROM pg_dist_shard WHERE logicalrelid='create_distributed_table_non_empty_failure.test_table'::regclass;