Update ui_test_guideline.md

Author: alppeh

ui_test_guideline/ui_test_guideline.md

@@ -12,3 +12,4 @@
 - Emulator device's cpu/abi should be x86_64 to prevent "Process instrumentation crashed..." error when tests are running.
 - Window animation scale, Transition animation scale and Animator duration scale options should be setted as "Animation off" in the developer options to prevent crashes when the element is not found because of the animations.
 - Regression tests should be run with real-data on Prod server. All the other tests should be run with mock-data on Stage server. 
+- In order to run tests on Prod server, **com.trendyol.regressiontests=true** parameter should be added into **gradle.properties** file.