Update QEMU and FVP scripts for optee_test / xtest

Since we now have official support for xtest, we have updated our two
setup scripts for QEMU and FVP to make use of the new git called
optee_test.git.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (QEMU and FVP)
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>