Update tests to support imp fields in tee_client_api.h types

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>

host/xtest/regression_1000.c

@@ -2036,7 +2036,7 @@ static void xtest_tee_test_1025(ADBG_Case_t *c)
 
 	Do_ADBG_EndSubCase(c, "Invalid NULL buffer memref registration");
 
-	if (!xtest_teec_ctx.memref_null) {
+	if (!xtest_teec_ctx.imp.memref_null) {
 		Do_ADBG_Log("Skip subcases: MEMREF_NULL capability not supported");
 		return;
 	}