[iOS sim] Ensure simulator device is booted in iossim_prepare.py

Recent versions of the iOS simulator require that a "simulator device"
is booted before we can use `simctl spawn` (see iossim_run.py) to start
processes.

We can use `simctl bootstatus` to ensure that the simulator device
is booted before we run any tests via lit.  The `-b` option starts the
device if necessary.

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D71449