folly: fixup folly::Random FixedSeed test expectations on macOS
folly: fixup folly::Random FixedSeed test expectations on macOS

Summary:
uniform_int_distribution is implemented differently on this system and
always returns the generated number from our constant random number generator.

Reviewed By: yfeldblum

Differential Revision: D3585418

fbshipit-source-id: 754076599ba77b49c17f03bb0c92264a3bed4ab0

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