Add missing unit tests for `get_page_by_title()`

[desrosj]

While looking into the report in #46355, I noticed that there are no unit tests for get_page_by_title().

[birgire]

Looks good @desrosj and runs successfully on my install.

Just minor things I noticed in 46439.diff​:

• I think we could delete this comment in test_get_page_by_title():

  // get_page_by_path() should return a post of the requested type before returning an attachment.
  

• There's a global $wpdb; in test_get_page_by_title_with_special_characters() that's not needed.

• Would be nice to have the file header with title, @since 5.2.0, etc.

[desrosj]

Thanks for the review, @birgire! I dug a little deeper into Trac today and discovered #36905. That ticket has a unit test patch already and discusses whether to add caching. I am going to close this as a duplicate.

[birgire]

yeah good finding @desrosj, hopefully these can be merged later on, e.g. the tests related to ticket #46355 on special chars.