REGTESTS: converter: add a test for the string trimming and parsing converters

REGTESTS: converter: add a test for the string trimming and parsing converters

The ltrim, rtrim, cut_crlf, base2, port_only and language converters had
no coverage at all, and host_only was only used incidentally by other
tests. They are all checked here, including the corner cases of a string
entirely made of trimmed characters, of a host value without a port, of
an IPv6 host, and of an accept-language header matching nothing with and
without a default value. The "q=" test is also improved to test "q=0".

The input of cut_crlf comes from an URL-encoded query string parameter
since a header field may carry neither a CR nor a LF.