Remove Test button for integrations that have broken tests (have not defined a configuration test)

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This issue is part of the epic &9485. See that epic for more context.

About

After #385661 (closed) all remaining integrations (at time of writing) that have not defined their own #test method exhibit broken or useless configuration test behaviour #213123 (comment 1206699507).

Proposal

Switch off testing for all integrations that have not defined a configuration test themselves.

• Delete Integration#test, and expect children to implement it.
• Integration#testable? to return true if there is a #test method defined (and is project-level, until #213138)).