Online DDL: fix endtoend test dropping foreign key

[shlomi-noach]

Description

This is a quick followup to #14517

The onlineddl_scheduler test has an "invisible" failure, passing in OSS CI, but failing on a DROP FOREIGN KEY test running on publicly forked MySQL. While we are still to deploy the forked MySQL server in vitessio/vitess CI, here's a quick fix.

This fix works both as a standalone as well as in combination with #14517.

Related Issue(s)

• Online DDL: edit CONSTRAINT names in CREATE TABLE #14517
• https://bugs.mysql.com/bug.php?id=107772

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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