[tests] Skip MemoryStream_CapacityBoundaryChecks on CI due to large memory allocation

[kotlarmilos]

Description

This PR disables the MemoryStream_CapacityBoundaryChecks test with [Fact(Skip = "...")]. Previously, SkipOnCI still allowed test to run on Apple mobile platforms which caused failures.

[kotlarmilos]

/azp run runtime-extra-platforms

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[alinpahontu2912]

Lgtm, I'd just change the comment from skipping on CI to always skipping since it won't run locally now either

[vitek-karas]

I think the correct fix would be to do something about dotnet/xharness#1473. I looked into that some more and we probably don't want to change xharness itself, but instead modify how it's used from Arcade/Helix to pass the env. variables which denote CI runs to the tests.
There are now two tests in runtime which use the SkipOnCI attribute (for the same reason of allocating too much).

[kotlarmilos]

Sorry I missed that. Yes, that sounds reasonable. Let’s move the discussion there.