First off they will not test for steroids. Not for a 3rd degree misdemeanor. They will probably do randomized urine tests with a dipstick that tests for pot, opiates, amphetamines, cocaine, benzos and a couple of other things. The cost of the test is covered in your monthly probation fee. That is usually $40-60. Probation is a business in the sense that they try and save money too. A minor offense on alcohol does not warrant the expense and time to test for an unrelated compound. In certain states they will only test for alcohol via a tube with crystals you breathe into that changes colors if alcohol is present. Don't worry.
Even if they do test you will most likely not be in violation of the terms of your probation. Think of it this way. If you took a shot of Deca a year ago and they tested for steroids now do you really think they would lock you up? No. They might do blood testing to determine that the levels are consistent with your story, but all this is overkill.
Don't worry. Just stop taking it now. Start your PCT and get through your probation. Don't drink dont use anything while on. You will be fine. As long as you are not adjudicated guilty on this or the prior offense you can probably get your records expunged in the future. Just chill. And by the way lay off the booze. If you got arrested you were probably being a douche no matter how **** the cop is. Good luck!