Driver tests: set `--sysroot=""` to support clang with `DEFAULT_SYSROOT`

When testing clang that has been compiled with `-DDEFAULT_SYSROOT` set to some path,
some tests would fail. Override sysroot to be empty string for the tests to succeed
when clang is configured with `DEFAULT_SYSROOT`.

Differential Revision: https://reviews.llvm.org/D66834

Patch by Sergej Jaskiewicz <jaskiewiczs@icloud.com>.

llvm-svn: 373147