Fix unit tests for 6.4

[desrosj]

This backports the following to restore a passing build in the 6.7 branch:

• b6e708d for fixing the TLS/SSL connection issue in newer PHP 8.x containers (see Core-63876).
• 9ca38ce to skip add media tests that currently fail on ImageMagick 7.x, and improves error handling within tests (see Core-63932).
• A change to checkout a specific version of the WordPress Importer plugin on PHP < 7.2. Support for these versions has been dropped (Core-63983)

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[desrosj]

Fixed with r60749 and 460750