Infringement is defined as the unauthorized "use" of intellectual property, not the unauthorized "sale" of the goods. So just using the trademarked or copyrighted material without permission - even for personal use - is just as illegal as if you sold a million shirts. The laws do not change based on the amount of money involved.
That said, part of the damages collected by the IP owner in court could be based on money made by the infringing party. So if you only used it for personal use, damages could be very little. The IP owner, of course, could claim dilution of the value of their IP. And then it's up to the judge how much of an example they want to make of you.
And keep in mind, if your "personal use" shirt is considered a knockoff instead of infringement, then that's a whole different story. Counterfeiting is a felony and carries fines, seizure of your goods and equipment and potential jail time.
It's a common misconception that printing a one-off is somehow legal. Reality is, it's not. At best, it may be low risk because IP owners may never know about it. But there are no laws that void an IP owners rights to their work simply because you only print one shirt for yourself or a friend.
