[APFloat] Added unittest for APFloat.subtract that checks special cases, result categories, and result status.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185043 91177308-0d34-0410-b5e6-96231b3b80d8
[APFloat] Added unittest for APFloat.multiply that checks special cases, result categories, and result status.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185044 91177308-0d34-0410-b5e6-96231b3b80d8