[html-tests] Replace usage of setTimeout with step_timeout in html/syntax (#4982)

This is expected to help test stability on slow configurations, as the
timeouts will be multiplied by the timeout multiplier.

Split out from #1816.

View on GitHub: https://github.com/w3c/web-platform-tests/pull/4982

Received on Wednesday, 22 February 2017 17:17:03 UTC