internal/screentest: reduce motion on web pages
internal/screentest: reduce motion on web pages

Added a stylesheet insertion step that reduces
motion on web pages by speeding up animations
and removing animation delays.

Change-Id: If945f764dfafaebfedce87fa462c5824033808e2
Reviewed-on: https://go-review.googlesource.com/c/website/+/379134
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Trust: Jamal Carvalho <19dd88ea049b99d7eb75a23cc7da743ca525bca4@google.com>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website