Driver tests: remove `REQUIRES: x86-registered-target` and set `--sysroot=""`...

Driver tests: remove `REQUIRES: x86-registered-target` and set `--sysroot=""` to support clang with `DEFAULT_SYSROOT`.

When testing clang that has been compiled with -DDEFAULT_SYSROOT set to some path, ps4-ps5-header-search.c would fail.

The test needs to be updated.

    1. Remove unnecessary REQUIRES: x86-registered-target.
    2. Override sysroot to be empty string for the test to succeed when clang is configured with DEFAULT_SYSROOT.