regression_1025: use the imp field in TEEC_Context

As the test xtest_tee_test_1025() uses a part of a struct from
tee_client_api.h that is implementation defined, the test
needed to be updated when the implementation defined parts of
the structs in tee_client_api.h changed to use an imp field
instead of separate fields.

Signed-off-by: Julianus Larson <julianus.larson@linaro.org>
Reported-by: Tom Hebb <tommyhebb@gmail.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>