Also add a more robust set of unit tests for this internal class.
Because the GL spec allows for succeess when packing any set of
varyings, it's impossible to write negative end to end tests. This CL
also rewrites our disabled varying packing GLSL tests as unit tests.

BUG=angleproject:1296
BUG=angleproject:1638

Change-Id: I78153742517d5c72ddb13ff59dc44ddc4af42fc2
Reviewed-on: https://chromium-review.googlesource.com/415555
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>