Tests failing on WP 3.7 due to PHP minimum version support issue

[schlessera]

Currently, tests on WP 3.7 are failing, because of an update to the wordpress-importer plugin that bumped the minimum PHP requirement to 5.6+: https://github.com/wp-cli/extension-command/actions/runs/3269158784/jobs/5376377280#step:13:39

Due to https://meta.trac.wordpress.org/ticket/6292, WP-CLI cannot retrieve an update anymore for WP 3.7.

As it is unlikely for the above Meta issue to get resolved anytime soon, the tests will need to be rewritten with a different plugin that will still allow for all scenarios to work across all supported WP versions.