Support: BlockFrequencyTest => BranchProbabilityTest

Move a detailed test of `BranchProbability::scale()` from
`BlockFrequencyTest` over to `BranchProbabilityTest`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207552 91177308-0d34-0410-b5e6-96231b3b80d8