prevent test/reset plug from tripping when electrical current stops?
Does your heater or your timer have a ground fault? You can test for that. The test/reset plug is called a Ground Fault Current Interrupter, or GFCI. The GFCI works by monitoring current flow in the load and trips if it sees an imbalance. The ‘imbalance’ is assumed to be current flowing to ground instead of the return line.

Test 1 - heater by itself. Plug it in to the GFCI and try turning it on and off.

Test 2 - timer. Try the timer with a known load - use a lamp or even better, a high-watt load like a toaster, electric kettle, etc.

Test 3 - known load by itself. Use that high-watt load from test 2.

If all 3 fail, chances are the GFCI itself is faulty. They’re inexpensive and easy to replace. If only 1 or 2 fail, suspect the heater or timer, respectively. 

I would not recommend replacing the GFCI with an ordinary receptacle. A garage is a ‘wet’ location - like outdoor - so the GFCI is providing an extra measure of safety.

In 120V countries, a newer type of protection is available: the Arc Fault Current Interrupter (AFCI), which detects abnormal spikes in current characteristic of arcing. This is available as a breaker, or as a dual-mode GFCI/AFCI receptacle. Something to consider if you’re replacing the receptacle (and a very good idea for a garage plug.)