[PECOFF] Remove a redundant test.

peplus.test and pe32plus.test basically covered the same thing, so
remove pe32plus.test and then rename peplus.test -> pe32plus.test.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200275 91177308-0d34-0410-b5e6-96231b3b80d8