test: Add missing test table cleanup for MaterializedViewIT

[trollyxia]

Change-Id: I1281568894ac2d04f0aafe0aa7f4efa7570ae58a

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)
• Rollback plan is reviewed and LGTMed
• All new data plane features have a completed end to end testing plan

Fixes #<issue_number_goes_here> ☕️

If you write sample code, please follow the samples format.

[gcf-merge-on-green]

Merge-on-green attempted to merge your PR for 6 hours, but it was not mergeable because either one of your required status checks failed, one of your required reviews was not approved, or there is a do not merge label. Learn more about your required status checks here: https://help.github.com/en/github/administering-a-repository/enabling-required-status-checks. You can remove and reapply the label to re-run the bot.