Rewrite and organize tests in LayoutTests/shadow-dom

To separate our concerns, move some tests to LayoutTests/shadow-dom/v0 directory if
they are also testing v0 features for compatibility reasons.

In summary:

- LayoutTests/shadow-dom/
  Used for tests if they are testing only v1 features
- LayoutTests/shadow-dom/v0/
  Used for tests if they are also testing v0 featuers

Clean up is not finished. Other clean up CLs will come later.

BUG=531990

Review-Url: https://codereview.chromium.org/2051373002
Cr-Commit-Position: refs/heads/master@{#399127}
24 files changed