Derive w3c_known_failures.txt from EXPECTED_FAILURE()

[Chris Hillery]

Having two sources of known-failing XQTS tests is error-prone. In particular, because the remote queue depends on w3c_known_failures.txt, it does not test that EXPECTED_FAILURE()s are correctly updated when things are fixed.

We should build w3c_known_failures.txt out of the EXPECTED_FAILURE()s list, and eliminate the auto-progression mechanism on the remote queue.