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I normally swap out a the tube on the trail if I get a flat and then patch at home. So that leaves me with a patched tube ready to put in a tire and I'd like to test it without having to swap out a tube ('cause I'm lazy :).
I thought that perhaps I could just inflate it but I'm concerned that without the opposing pressure of the tire it might blow an otherwise solid patch.
I do like you do when riding, and I usually save up my tubes with holes and patch a bunch of them all at once. That way I can use a tub of water to both find the holes, and can go back through them after patching and test to see if they are holding air.
If I have any doubt after patching a tube, I give it a little time to cure and then I pump it up and hang it from a hook in my garage overnight to see if it loses air.
You should inflate the tube to 1.5 to 2 times the "normal" diameter, in order to develop a modicum of pressure. (Wait until after the patch has "cured" overnight to do this, however.) Then either test in a tub/sink or let it sit overnight again to see if it loses air.
(Getting the tube to fold up nicely is not a problem, if it's a Presta. Just squeegee out all the air, close the valve, then lay it flat and fold it up. Schrader is harder -- best is to remove the valve core temporarily while getting the air out.)
You have the right idea. Inflate the tube to roughly the side it is in the tyre, which will be a much lower pressure than it would be if it was in a tyre. My floor pump doesn't register the pressure of a tube pumped up like this, so I expect it's less than 10psi/1 bar.
A properly applied patch should resist inflating the tube up to 1.5-2 times the nominal diameter. This is useful for testing the quality of the patching work but also to find the tiniest holes that sometimes are harder to spot.
As other say, the definitive way to test is to inflate and submerge in water or to inflate and let overnight to see if it holds.
However: I you get these tiny punctures and specially if they develop on the inner side of the tube (or "high" on the face sides), then the problem may be a faulty or degraded tube. Tubes may suffer from chemical reactions which makes them prone to develop spontaneous pores. When such moment arrives it is best to buy a new tube (one that has not been in the shelves for too long).
Some other ways to spot a chemically damaged or degraded tube is that when inflating them outside the tire, they have irregular shape (thicker in one parts than other) or the valve spins towards the outside. Other may show cracked or rough spots (or other odd texture). Another symptom may be the tube looks "too black" (a normal tube is actually dark gray) and they feel sticky.
My experience is that you can't fully test a patch until you take it up to full pressure. I had a patch that was fine for months at low pressure on the bike. I took the 2.10 up to full pressure to ride on the road and the patch failed. Even if you put it on the bike to test the problem is getting the fresh tube you used back in the seat bag. I only use patched tubes for repair at home.
