x86 UEFI: Improve Boot Speed

The previous runner script uses `startup.nsh` to specify the EFI
binaries. When a UEFI firmware sees this script, it waits 5 seconds for
user input before executing the script.

To remove this 5-second waiting, this commit drops the `startup.nsh`
script, and renames test case binaries as UEFI's default binary filename
`EFI/BOOT/BOOTX64.EFI`. This does not trigger the 5-second waiting.

This commit only fix the boot speed for x86/efi/run script. The
run-tests.sh will be fixed by the follow-up patch.

Signed-off-by: Zixuan Wang <zxwang42@gmail.com>
Message-Id: <20211116204053.220523-10-zxwang42@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>