[infra-tests] use context manager instead of prerun script (#5771)

This addresses feedback on https://github.com/w3c/wptrunner/pull/253 that was merged without this enhancement (fixes https://github.com/w3c/wptrunner/issues/255)

View on GitHub: https://github.com/w3c/web-platform-tests/pull/5771

Received on Wednesday, 3 May 2017 15:31:00 UTC