Move tests for ms_files_rewriting to separate group, ms-files

When the `ms_files_rewriting` flag is enabled, `ms_upload_constants()` is required to properly set upload directory constants. Once this fires, it is impossible to clean up for a non `ms_files_rewriting` test by turning the option back off.

Excluding these tests by default offer a more consistent environment overall. Any tests written for uploaded files in multisite should ideally have a correspondign test in this area.

This commit also moves existing `ms_files_rewriting` tests for `test_switch_upload_dir()`.

Fixes #30256


git-svn-id: https://develop.svn.wordpress.org/trunk@30286 602fd350-edb4-49c9-b593-d223f7449a82
Fix incorrect documentation in the DocBlocks for `get_lastpostmodified()` and `_get_last_post_time()`.

Props ericlewis for the initial patch.
Fixes #30249.


git-svn-id: https://develop.svn.wordpress.org/trunk@30287 602fd350-edb4-49c9-b593-d223f7449a82