Posts, Post Types: Don't force trailing slashes in get_pagenum_link().

[costdev]

Previously, a trailing slash was appended to the link returned from this function. If the permalink structure didn't contain a trailing slash, this link could fail.

This change removes trailing slashes and only appends one if the site is set for adding trailing slashes.

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This PR is an update of #3931 which makes various test improvements, including:

• Documentation updates.
• Backing up $_SERVER['REQUEST_URI'] before any tests run, and restoring it after each test has run.
• Using a data provider to group related datasets.
• Adding datasets that account for index.php in the permalink structure.
• Adding datasets that account for plain permalinks, date-based permalinks, and postname-based permalinks.
• Adding a test that verifies that get_pagenum_link() adds a trailing slash when the permalink structure contains a trailing slash.

Trac ticket: https://core.trac.wordpress.org/ticket/2877

[costdev]

Committed in r56939.