test: add extremely robust DeferJavascript test coverage

- Created 17 comprehensive tests with 47 assertions
- Documents GLightbox scenario from issue #173
- Tests all edge cases: type attributes, async, integrity, crossorigin
- Tests JSON-LD, module scripts, nomodule, empty src
- Tests relative/absolute paths, protocol-relative URLs
- Tests single/double quotes and no quotes
- Performance test: 100 scripts in <100ms
- All 62 tests passing with 246 assertions
- Relates to #173

Author: renatomarinho