[2dcontext-tests] Remove synchronous forks of drawImage tests, and enable proper asynchronous drawImage tests. (#5530)

Upstreamed from https://github.com/servo/servo/pull/14962 [ci skip]

View on GitHub: https://github.com/w3c/web-platform-tests/pull/5530

Received on Tuesday, 11 April 2017 06:35:00 UTC