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 <firstname.lastname@example.org>.