WOFF2 spec and CTS updates

Folks,

In preparation for the discussions at our telcon on Wednesday and upcoming F2F next week I made the following changes:

-          WOFF2 spec - added a new UA conformance requirement in the end of the section 4.2 "Collection directory format" [1] and its description in UA test plan [2] to address open action 184;

-          Updated the language of the hmtx transform to disallow null transform [3];

-          Added new test cases [4-7] covering new conformance requirements for 'hmtx' table transform;

At this point, I think that we at least have full CTS coverage as far as test plans vs. spec are concerned and should be able to finalize all the descriptions and update the status of the existing tests at the F2F.

Looking forward to our discussions,
Vlad

[1] http://dev.w3.org/webfonts/WOFF2/spec/#collection_dir_format
[2] https://www.w3.org/Fonts/WG/wiki/TestPlan20-UserAgent#mustLoadFontCollection
[3] http://dev.w3.org/webfonts/WOFF2/spec/#hmtx_table_format
[4] https://www.w3.org/Fonts/WG/wiki/TestPlan20-UserAgent#mustCheckLSBFlags
[5] https://www.w3.org/Fonts/WG/wiki/TestPlan20-Format#transformFlagsMustBeSet
[6] https://www.w3.org/Fonts/WG/wiki/TestPlan20-AuthoringTool#mustCheckLeftSideBearings
[7] https://www.w3.org/Fonts/WG/wiki/TestPlan20-Decoder#mustReconstructLSBs