Add conformance test script

[gyagp]

This script is to run any version of Khronos WebGL conformance test on
various OSes (like Android, ChromeOS, Linux, MacOS and Windows) with
various browsers (like Chrome, Edge, FireFox, Safari, etc.). Results
will be compared with expectations and a final report will be generated.

[gyagp]

@kenrussell, sorry that this has taken a bit longer as I almost rewrote my script, added more features, and enhanced the performance. Now all the main OSes, including Android, ChromeOS, Linux, MacOS and Windows, have at least support of 1 or 2 browsers, including Chrome, Edge, FireFox and Safari. For sure, there are many todos, but I think it's already good enough for first cut. For more details, please see the README.md enclosed.
Please suggest on folder and file names, on which I don't have a good idea yet. And now I put all the things in a single script, which might be handy for testing, but also be evil for code maintenance.
Please also comment on the detailed code and some general issues like coding style, etc.
Thanks!

[kenrussell]

Yang, thanks for this tremendous contribution and I'm very sorry for sitting on it so long unreviewed. Finally set it up locally and ran it; it's working well. Let's do further improvements as follow-on pull requests.

For the record, was able to launch tests in Chrome via:

python conformance.py --browser-path /Applications/Google\ Chrome\ Canary.app/Contents/MacOS/Google\ Chrome\ Canary --browser-name Chrome --suite conformance/attribs

Merging now. Thanks again.

[gyagp]

Thanks for the merge!