shadow-dom: Reorganize the test structure

This pull request basically converts the tests under the shadow-dom/ directory into separate HTMLs, in order to have better alignment with other test suites in web-platform-tests.

This branch consists of three commits:
(1) a change adding a script named convert.py that does the conversion;
(2) the actual conversion (note: this change is huge, so it will take some time to load on GitHub UI);
(3) a change removing convert.py.

If you are interested in what is the actual change, please consult the content of convert.py. I did not make any manual changes in the commit (2), except for moving the content of the output directory of the script.