set CFG_TA_TEST_PATH=y for optee_client build

With CFG_TA_TEST_PATH now defaulting to n in optee_client for better
security, this needs to be set explicitly to pass xtest 1008.

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
diff --git a/common.mk b/common.mk
index a34e0e0..00d6146 100644
--- a/common.mk
+++ b/common.mk
@@ -404,6 +404,7 @@
 
 OPTEE_CLIENT_COMMON_FLAGS ?= CROSS_COMPILE=$(CROSS_COMPILE_NS_USER) \
 	CFG_TEE_BENCHMARK=$(CFG_TEE_BENCHMARK) \
+	CFG_TA_TEST_PATH=y
 
 .PHONY: optee-client-common
 optee-client-common: