Bug 980934 (part 1) - Add mfbt/tests/TestSplayTree.cpp. r=froydnj.

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extra : rebase_source : 2fa739b40d6b6f136002abc875a7c558cb4dd4ed
Bug 980934 (part 2) - MFBT: Remove the excessively frequent coherency checks from SplayTree. r=froydnj.

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extra : rebase_source : 4d0bd8e2d56771ee8ee4612ce3536bbdc23e958e