Warning for invalid Tested up to value

[ernilambar]

Fixes #609

[github-actions]

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[swissspidy]

Could we add tests for this? :-)

• Latest WP version
• Latest +1
• Latest +2

[ernilambar]

@swissspidy I was thinking about using similar test like test_run_with_errors_parser_warnings_with_custom_set_transient_version(). Should we add three separate test plugins for this? Or is there any better approach?

[swissspidy]

Mocking the transient makes sense to me. We have way too many test plugins already 😄

[ernilambar]

Unit test added.