Remove Importer plugin tests

[desrosj]

This removes the tests from Core that rely on the presence of the WordPress Importer.

These have been running in https://github.com/WordPress/wordpress-importer for several years now, and it eliminates the need to download the plugin before every run of the Core test suite.

In order to ensure the tests continue to pass, this also adds a weekly schedule event.

Trac ticket: https://core.trac.wordpress.org/ticket/42668

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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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[github-actions]

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Props desrosj, swissspidy.

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[swissspidy]

Makes sense to me. Will reduce some confusion for contributors when running these tests.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 59769
GitHub commit: 5b5d358

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.