[tests] Remove use of dynamic property Parser::$static_tag_buf

[tests] Remove use of dynamic property Parser::$static_tag_buf

There are only two test cases using the <statictag> tag extension and
it's no longer entirely clear what regression they were intended to
prevent.

In any case, we can use ParserOutput to maintain our parser state
instead of the dynamic Parser::$static_tag_buf property.  That's not a
perfect solution -- T300979 says at some point in the future we won't
guarantee parse order -- but it will work for now.

Bug: T357838
Change-Id: Ia23049fc729c90a94fc16231ad89c199db5b7bc9