Definition of testing grounds:
(n) :
(figuratively) A situation or thing that allows new ideas, methods, or hypotheses to be tried out.
(n) :
A location set up for the testing of vehicles, machinery, agricultural techniques, etc. to ensure that they work as intended.
(n) :
A military site that is used for testing weapons.
(n) :
(figuratively) A situation or problem that allows people to test how good they are.