Fix accidentally removed test.

When adding the new tests for the filesystem_error::what method,
I incorrectly removed a test case and replaced it with something else.

This patch restores that test case

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337764 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed