Remove --fuzzy-diff from Vulkan swiftshader web tests

Improve the accuracy of this test suite by removing --fuzzy-diff and
adding exact baseline images.

These tests are run on the CQ by default and don't rely on specific
hardware, so they can be rebaselined locally using Linux.

Bug: 1243824
Change-Id: I92ab413bfc80db4c888b310d71900b2b8e3a0165
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3138355
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: River Gilhuly <rivr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#919024}