Unit Tests: Speed up multisite specific test

[r-a-y]

One of our multisite unit tests was slowing things down considerably - test_bp_core_get_directory_pages_multisite_delete_post_with_same_bp_page_id().

I introduced this in #6226. Basically, the test would create new posts in a loop(!) until it reached a certain page ID on a sub-site.

There is a much more efficient way of doing this by using a not-well-documented parameter in wp_insert_post() called 'import_id'. Setting 'import_id' will just use the post ID we want to use.

Going to commit.

[r-a-y]

In 11880:

Unit Tests: Improve performance of multisite directory page test.

The unit test in question was introduced in #6226.

Basically, the test needed to match a specific WordPress post ID.
Previously, the test did this by creating posts in a loop(!) until it
matched the post ID. However, there is a parameter in wp_insert_post()
that allows you to set the post ID from the get-go - 'import_id'.

This commit switches to using the 'import_id' parameter to create the
WordPress post and should speed up this test considerably.

Fixes #7709.