Adds an Expect script to launch QEMU, run xtest and return a pass/fail
status. This makes it even easier to launch non-regression tests and
possibly integrate into a CI loop.

The script may be invoked through 'make check' or 'make check-only'.
The former considers dependencies, while the latter just runs the script.

The output of serial ports 0 and 1 are saved to serial0.log and
serial1.log, respectively.

Expect may be installed with 'sudo apt-get install expect' on Debian/
Ubuntu distributions.

Sample output:

 $ make -j9 check
 <snip>
 expect qemu-check.exp -- \
         --bios /home/jerome/work/optee_repo/out/bios-qemu/bios.bin
 Starting QEMU... done, guest is booted.
 Loading OP-TEE driver and tee-supplicant... done.
 Running: xtest...
 ##########################################<snip>
 Status: PASS (404 test cases)

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>