You will just have to rope drop from the International Gateway. You can still make it to Test Track, you just won't be at the front of the line. Your other option is to Uber/Lyft/taxi to the front.
UNLESS: Does anyone know if they're back to running buses from DHS to Epcot? I know they were doing that pre-COVID, even after the Skyliner opened. If they are, you could Skyline over to DHS and catch a bus to the front, but that could be a VERY slow option. You might be stuck waiting for a bus for a long time, so I wouldn't recommend this option.
My advice would be to just rope drop from IG and hustle over to Test Track. If the single rider line is open, you can use that and ride a lot quicker that way. You'll skip the design-a-car portion, but it can be a much quicker ride first thing in the morning. I rope dropped TT once with my 2 teens and we went straight to the single rider and got right on and even rode together because the first group through the regular line were still working on their car designs.