[test] refactor serialization tests

- combine item-separator with method tests into one
  `ser:item-separator-with-method`
- simplify test cases that used eq in combination with assertXPath
- make direct calls to function under test (serialize) in each test
- extract fixtures to module variables
- remove namespace from children of output:serialization-parameters
- convert QNames keys to strings for exist-db specific options
- convert string constructors to CDATA sections
- make HTML5 test data real world examples
- use unprefixed form of built-ins
- fix formatting