Replace superclass with _TestBase variant.

List which issues are fixed by this PR. You must list at least one issue.

Pre-launch Checklist

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• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
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• All existing and new tests are passing.