Fix reftest for parallel test runs.

This adds a sha1 hash to the output filenames based on the test. Rust runs
tests in parallel by default, so the test files were stomping on each other.
auto merge of #610 : metajack/servo/fix-reftest, r=pcwalton

This adds a sha1 hash to the output filenames based on the test. Rust runs
tests in parallel by default, so the test files were stomping on each other.