[tests][Driver] Set --sysroot="" to allow DEFAULT_SYSROOT build
If DEFAULT_SYSROOT is configured to some path, some tests would fail.
This patch overrides sysroot to be the empty string in the style of
D66834 so that the tests will pass even when the build is configured
with a DEFAULT_SYSROOT.
Reviewed By: mstorsjo
Differential Revision: https://reviews.llvm.org/D79694