COFF: fix an off by one error

Adjust the tests to validate the number of auxiliary entries used to store the
filename.

Thanks to majnemer's sharp eye for catching the missing - 1 in the round up
calculation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206359 91177308-0d34-0410-b5e6-96231b3b80d8