Tests/Importer: don't fail tests which should be skipped

[jrfnl]

Tests which can not run because a prerequisite is not fulfilled should be marked as "skipped", not as failed (as they haven't failed, they didn't run).

Now, I can imagine the choice to mark the tests as "failed" may have been to prevent contributors missing the message about the need to install the Importer plugin. If that's the case, I'd suggest adding a file_exists() check for the importer plugin to the test bootstrap.php file and exiting the test run if that prerequisite is not fulfilled, along the same lines as the check that the database connection can be made and that the correct version of the Polyfills is installed.

I'll happily update the test bootstrap to add this, but would like some more opinions on whether or not this is really a necessity.

Trac ticket: https://core.trac.wordpress.org/ticket/61530

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[SergeyBiryukov]

Thanks for the PR! Merged in r59085.

[jrfnl]

Thanks @SergeyBiryukov! And thanks for adding the WP_RUN_CORE_TESTS check to the condition too.