torture: Flag console.log file to prevent holdovers from earlier runs

torture: Flag console.log file to prevent holdovers from earlier runs

A system misconfiguration that prevents qemu from running at all (for
example, a missing dynamically linked library) will keep the console.log
file from the previous run.  This can fool the developer into thinking
that this failed run actually completed correctly.  This commit therefore
overwrites the console.log file just before launching qemu.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>