x86 UEFI: Make run_tests.sh (mostly) work under UEFI

Make several fixes so that run_tests.sh can run the test cases when
the repo is configured to run under UEFI. Specifically:
1. The scripts are enlighted to run the .efi test cases rather than the
.flat test cases.
2. The scripts are enlighted to run an empty file, _NO_FILE_4Uhere_,
which is used to sanity check QEMU and QEMU flags.
3. The scripts are updated to always run with `-smp 1`, since SMP is not
yet supported under UEFI.

Notably, QEMU's `-append` flag still does not work. This will need to be
fixed in future commits. For now, test cases that use `-append` are
marked `SKIP`.

Signed-off-by: Marc Orr <marcorr@google.com>
Message-Id: <20211116204053.220523-9-zxwang42@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>