"Testing started" versus "testing has been started"

I wish to notify my colleagues that they may start testing because I have prepared the environment. Among the many ways to say this, I wonder whether I can say simply Testing started or Testing has been started.

I feel that passive voice is not appropriate here, but others vote for it.

Which one would be the correct version: one of these two or a different one?

Intuitively, both your propositions do not say what you intend. If you wanted to inform the customer that your colleagues have started testing or that the testing phase has begun, both would be OK. Passive voice would then emphasize the fact that there was an (undisclosed) actor involved.

To express availability of the test environment I would say "Testing may be started".