After the first test using the Mock ICD runs, an environment
variable VK_ICD_FILENAMES is set when previously it did not
exists. The cleanup of that test, rather than deleting the
environment variable, set it to an empty string, which the
Vulkan loader would use for subsequent Vulkan tests, causing
issues with Vulkan not finding the correct extensions.

Had to disable two more tests since they started failing once
those backends were enabled.

Bug: angleproject:2604
Change-Id: Ie103756eed60d255d1dd92d7ef3b726183feeb1e
Reviewed-on: https://chromium-review.googlesource.com/1137342Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Omar El Sheikh <theoking@google.com>