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