tests/benchmarks/boot_time: disable on QEMU targets

These all currently use the HPET timer, and thus can't build with
BOOT_TIME_MEASUREMENT enabled, so disable for now. This test is
basically a build-only test, so we're covered with the other x86
targets (gpmrb, up_squared) for now.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>