test(pgsql): register retry_guard_in_txn_on_broken_backend in groups.json

The test added in 8e509d371 was not registered in groups.json, so no TAP
group was running it. Register it under legacy-g2.

test/tap/groups/groups.json

@@ -148,6 +148,7 @@
   "pgsql-reg_test_5300_threshold_resultset_deadlock-t" : [ "legacy-g4","mysql-auto_increment_delay_multiplex=0-g4","mysql-multiplexing=false-g4","mysql-query_digests=0-g4","mysql-query_digests_keep_comment=1-g4" ],
   "pgsql-reg_test_5352_prepared_statement_refcount_race-t" : [ "legacy-g4","mysql-auto_increment_delay_multiplex=0-g4","mysql-multiplexing=false-g4","mysql-query_digests=0-g4","mysql-query_digests_keep_comment=1-g4" ],
   "pgsql-reg_test_5415_copy_error_recovery-t" : [ "legacy-g4","mysql-auto_increment_delay_multiplex=0-g4","mysql-multiplexing=false-g4","mysql-query_digests=0-g4","mysql-query_digests_keep_comment=1-g4" ],
+  "pgsql-retry_guard_in_txn_on_broken_backend-t" : [ "legacy-g2" ],
   "pgsql-servers_ssl_params-t" : [ "legacy-g4","mysql-auto_increment_delay_multiplex=0-g4","mysql-multiplexing=false-g4","mysql-query_digests=0-g4","mysql-query_digests_keep_comment=1-g4" ],
   "pgsql-set_parameter_validation_test-t" : [ "legacy-g1","mysql-auto_increment_delay_multiplex=0-g1","mysql-multiplexing=false-g1","mysql-query_digests=0-g1","mysql-query_digests_keep_comment=1-g1" ],
   "pgsql-set_statement_test-t" : [ "legacy-g4","mysql-auto_increment_delay_multiplex=0-g4","mysql-multiplexing=false-g4","mysql-query_digests=0-g4","mysql-query_digests_keep_comment=1-g4" ],