oh sorry, i sometimes forget that not everyone is going to understand my personal slang.

"testing the water" simply means push a little and see if you get in trouble, and then when you don't push as little bit more and a little bit more to see how much you can get away with.

in this instance it just means that people are wandering a bit further into areas each day that the government says they are not supposed to access during the lockdown such as loads of people are going to the beach or the streets to exercise at night even though this is one of the many restricted activities.