ta: os_test: Unmask cancellation from invoke command handler

The specification seems to indicate (see https://github.com/OP-TEE/optee_test/issues/731

)
that the cancellation mask is reset at the start of every entry point.
As a result, the TEE_UnmakCancellation() should not be called at the end of
TA_OpenSessionEntryPoint(), but at the start of TA_InvokeCommandEntryPoint().

Signed-off-by: Cedric Auger <cedric.auger@provenrun.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@foss.st.com>