Adds --deqp-egl-display-type=angle-metal for running the deqp tests
on Mac. Note: does not add any test suppressions for metal.

Bug: angleproject:2634
Change-Id: I8f8d3a83cf93a5b930b05af9f0075a9ce435823a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1954612Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>