Remove WordPress Importer tests from default test suite

[Frank Klein]

The importers were removed from Core in r14764, 8 years ago. But the tests are still located in the core test suite, and are run by default.

We should exclude these tests from running by default as a first step, as it means running 14 tests less.

Ultimately these tests should be migrated to the ​Importer plugin repository, where they can assist with development.

[netweb]

The wordpress-importer import plugin is imported into our Travis CI setup and the tests are run in each CI job

See: https://core.trac.wordpress.org/browser/trunk/.travis.yml#L41

[dd32]

FWIW These should continue to be run in core until ​https://github.com/WordPress/wordpress-importer/issues/15 is fixed to move them over there IMHO.

Even then, running the tests with core might still be a good idea, although limiting it to 1 job instead of 5 wouldn't be a bad idea.