Disable all SDK snapshot generation tests on mac.

We frequently miss putting in the per-test skip condition, and since
mac isn't tested in presubmit this leads to build breakages and time
consuming reverts. To avoid that this blanket disables all the SDK
tests on mac. It's not used there and we got test coverage in many
linux-based builds.

Bug: 145598135
Bug: 161315642
Test: `go test -v ./sdk` in build/soong on mac and linux
Change-Id: I2aea92fef2c0f8c2742396fe36610501dc5a6f0f