The test perfschema.transaction_gtid executes transactions and checks
that the GTID field of
performance_schema.events_transactions_[current|history] is as
expected.

The following statements did not add any row to these tables before
WL#6378, but changed in WL#6378 so that they add a row to these
tables:

- TRUNCATE
- SELECT stored_function()
- SET AUTOCOMMIT

These statements were used in the test, and thus the change in the
worklog caused the test to fail with result mismatch.

We fix the TRUNCATE case by executing it on a different connection and
using a WHERE clause when reading the tables so that it only selects
rows from the connection where the transactions we are checking are
executed.

We fix the SELECT stored_function() case in the same
way. Additionally, we simplify the SELECT and use a .inc file instead
of a stored function.

We fix the SET AUTOCOMMIT case by switching the order between SET
AUTOCOMMIT and clearing the tables, so that the tables are cleared
after SET AUTOCOMMIT.

@mysql-test/include/execute_*_sync_point.inc
- Allow sourcing a file rather than executing a statement on the
  auxiliary connection.

@mysql-test/suite/perfschema/include/reset_transaction_gtid.test
- Use the auxiliary connection to clear the tables rather than
  the connection where transactions are executed.
- Since we use the correct connection, no need to set GTID_NEXT.

@mysql-test/suite/perfschema/include/show_transaction_gtid.test
- Avoid using a stored function.
- Document the file.

@mysql-test/suite/perfschema/t/transaction_gtid.test
- Use have_debug_sync.inc, not have_debug.inc.
- Remove the now unused stored functions.
- Invoke reset_transaction_gtid.inc after SET AUTOCOMMIT.