* More informative rewrite test output

When running rewrite tests, the output in case of a test failure now
includes not only the rewritten URLs but also the from URL.

* Move re-escaping to regexpMatches

This commit moves the code to post-process the match replacements from
ComplexRule to regexpMatches, so it can later be re-used for SimpleRule.

Also changes the comment in an attempt to better explain the reasoning
behind that code.

The required strings.Replacer is now built only once.

* Support regex captures in simple rewrite rules

Closes #2586