Add unit tests for global-struct initialization in Metal.
Add unit tests for global-struct initialization in Metal.

Previously, we had no unit test coverage for the Metal `struct Globals`
object. This CL adds simple tests for numeric globals and samplers.

Change-Id: I259c3cf416d5a1a03b1f815cc4c03891d60cbd08
Bug: skia:10382
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/300616
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Commit-Queue: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia