What to do if I was working on my boss’s vehicle and finally got it running so I decided to test drive it without permission but rolled it?

Unuthorized use of a motor vehicle (or UUMV) is considered a state jail felony in Texas.  A state jail felony is punishable by not less than 180 days in state jail, but no more than two years.  Even if you had not rolled the vehicle, you could have been charged with UUMV because you didn't have permission to drive it.

So-- that's the scary news.  The positive news is that if you have never been convicted of a felony, you can enter a plea bargain for a sentence other than prison time.  This type of offense is eligible for deferred adjudication and regular probation.  State jail offenses can also be reduced and pled down to misdmeanors. 

From here, you have a couple of different options.  One, is to simply work out the best deal you can get.  This was an accident.  If you are willing to pay restitution or fix the vehicle, then you will get a much lighter sentence.  In these types of cases, where everyone knows everyone and it was not intentionally conduct, some DA's offices will dismiss the charges if the victim is made whole with restitution. 

Your second option is to contest the charges.  I don't know of a mechanic in world that didn't test drive a vehicle around the block after they finished working on the vehicle.  You could make the argument that his consent was implied when he gave you consent to work on the vehicle (or asked you to work on the vehicle). 

In the end, the decision for which course of action you want to choose is up to you-- but generally, the "do right" rule is cheaper and a better guarantee.  If fixing the vehicle will only cost you $2000 or so, then it is probably cheaper and safer to ask for a conditional dismissal than paying an attorney a trial fee for a jury trial which has an unguaranteed result.  Restitution also helps preserve your relationship with your boss... assuming that you want to salvage it.