Partial fix for #16028 -- Moves tests for template filters from defaultfilters to templates.filters

Applies julien's patch 16028.WIP.diff, removing all u prefixes and replacing ur with r.
Removes duplicate filter test filter-urlizetrunc05

Also renumbers subsequent tests.
Removes mark_safe() call around filter-wordwrap03's context

Instead, uses the HTML entity for ' in the expected result.
Adds comments to clarify the intent of a couple of test cases

(Our understanding, anyway).
Replaces " with ' throughout, as appropriate.