Fix bugs with macOS test runner

[luispadron]

Fixes two major issues with the macos_unit_test runner

• Tests were always passing because:

xcodebuild <...> 2>&1 | tee -i "$testlog" \
    || test_exit_code=$?

Without pipefail means the exit code of the xcodebuild command is dropped. This meant as of #2649 all macos_unit_test were always passing even when tests failed 🙈

• Warning when running xcodebuild:

--- xcodebuild: WARNING: Using the first of multiple matching destinations:
{ platform:macOS, arch:arm64e, id:00006031-001851843684001C, name:My Mac }
{ platform:macOS, arch:arm64, id:00006031-001851843684001C, name:My Mac }
{ platform:macOS, arch:x86_64, id:00006031-001851843684001C, name:My Mac }
{ platform:macOS, arch:arm64e, variant:Mac Catalyst, id:00006031-001851843684001C, name:My Mac }
{ platform:macOS, arch:arm64, variant:Mac Catalyst, id:00006031-001851843684001C, name:My Mac }
{ platform:macOS, arch:x86_64, variant:Mac Catalyst, id:00006031-001851843684001C, name:My Mac }
{ platform:macOS, arch:arm64e, variant:DriverKit, id:00006031-001851843684001C, name:My Mac }
{ platform:macOS, arch:arm64, variant:DriverKit, id:00006031-001851843684001C, name:My Mac }
{ platform:macOS, arch:arm64e, variant:Designed for [iPad,iPhone], id:00006031-001851843684001C, name:My Mac }
{ platform:macOS, arch:arm64, variant:Designed for [iPad,iPhone], id:00006031-001851843684001C, name:My Mac }

The fix here is to specify variant=macos and arch so that xcodebuild always picks the right destination.

[adincebic]

Thanks @luispadron