Meaning of flickering light on outlet tester
Outlet testers indicate status by the combination of lights – “Correct” is just a combination of the two yellow/amber lights for “open ground” and “open neutral”. So yours is actually flickering between the “correct” and “open ground” states, meaning there is some sort of ground connection, just a very poor/unreliable one that shouldn’t be used.

This article from the Circuit Detective has an explanation of all the possible combinations of lights on a typical outlet tester, including dim/flickering ones.