Fix a compile failure introduced by r82675 on MinGW which doesn't have

Fix a compile failure introduced by r82675 on MinGW which doesn't have
setenv().  This patch just disables the test rather than getting putenv() to
work.  Thanks to Sandeep Patel for reporting the problem.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82797 91177308-0d34-0410-b5e6-96231b3b80d8