Add more tests for IndexedDB exception

 - rewrite the valid checkpoints from #292
Add more tests for IDBCursor update
Add a test for IDBFactory cmp
Use 'new TypeError()'
Add more tests for IDBIndex openCursor
Add more tests for IDBIndex openKeyCursor
Tests modified for IndexedDB
Rewrite manifest.py to make including local changes safer (not very backwards compatible)
Allow running tests from an external server root.

This is needed for supporting use cases where testsuites outside
wpt are run using the server in a wpt checkout. It also changes the
manifest format in a way that causes the file path to be derived from
the url unless otherwise specified.
Make img sizes parsing test match MQ spec, add more tests
Add a baseline test for 'or'
Clean up additions from @sideshowbarker
Merge pull request #1285 from w3c/fix-recent-urltestdata-additions

Clean up additions from @sideshowbarker
fixup review issues
Cleanup whitespace in MutationObserver-disconnect.html.
Step in the right test in MutationObserver-disconnect.html.
cc Renamed files (replace NNN-* with description).
cc Updated messages.json file.
Fix the README for url tests a bit.
Minor formatting tweak.