native_posix: Run in real time by default with host BT

For native_posix, set NATIVE_POSIX_SLOWDOWN_TO_REAL_TIME
by default when a host BT adapter is selected even if TEST
was also set. As using host peripherals one normally needs
also to run with the host time.

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>