A necesssary change in SwiftShader introduced a bug that was causing
test timeouts. Disable SwiftShader so they can revert the change
without breaking ANGLE.

Bug: angleproject:3876
Change-Id: I4b841a4a28fa13947c24b72d856db38c9e0135cb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1815908Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>