[tests][Driver] Set `--sysroot=""` to allow `DEFAULT_SYSROOT` build

Summary:
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