test: earlier assert rewrite for test utitlities

[aucampia]

Summary of changes

This patch moves the assert rewrite for test utilities to happen before
other imports to ensure that assert rewrites at the right time.

Without this some assert output were not formatted nicely with a diff
and just had AssertionError as output.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage remained the same at 88.238% when pulling 3f24fdf on aucampia:iwana-20220508T1517-early_rewrite into eba1373 on RDFLib:master.