URL encode filenames of uploaded test result files

[lukebjerring]

Test file paths should be url-encoded before being stored in the test results JSON.

Example: https://storage.googleapis.com/wptd/7a0cf8ade7/chrome-63.0-linux/html/syntax/parsing/html5lib_namespace-sensitivity.html?run_type=uri

Note that url-escaping the ? works correctly: https://storage.googleapis.com/wptd/7a0cf8ade7/chrome-63.0-linux/html/syntax/parsing/html5lib_namespace-sensitivity.html%2Erun_type=uri