This PR splits skipped tests into two categories:

• disabl,edTests: We eventually want to pass these, but right now we can't because either:
  • They crash workerd
  • The harness can't handle them
  • They test a feature we haven't implemented but might implement in the future.
• omittedTests: Do not include in our WPT report or coverage stats, because:
  • They're not reflective of an actual standard (e.g. most tentative tests)
  • They're not relevant server-side
  • We don't intend to support this feature
  • The file isn't a test but is picked up by our globs in bazel anyway
  • etc.

Please let me know if I've misclassified any of our existing tests.