Add failing tests from Erubi test suite

[joelhawksley]

As a fun experiment, I had Claude Opus 4.6 re-create the tests from https://github.com/jeremyevans/erubi/blob/master/test/test.rb inside Herb and see if there were any gaps. Of the 105 tests, Herb passed 97 and failed 8.

Category	Tests	Issue
<%% literal escape	3	Herb doesn't output <%% as literal <% text
trim: false	2	Herb strips whitespace around code/comment tags even when trim: false
Trailing content after %>	1	Herb drops leading whitespace when code tag has trailing content on same line
<% inside string	1	<% x = '<%' %> triggers Herb's HTML parser error
literal_prefix/literal_postfix	1	Custom literal prefix/postfix not supported

I only added the failing tests here, as I'm guessing many of the passing 97 would be duplicative with the existing suite.