qemu-check.exp: monitor TEE core log file and detect assertion failures

When "make check" is run and an assertion fails in TEE core, secure
world hangs, and therefore xtest hangs too. "make check" will not
report the failure and exit until a timeout occurs (480s).
In order to detect the error condition sooner, open the TEE core log
file and add a trigger on "assertion ... failed at ...".

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>