Trouble starting/low voltage

You can't use that test light trick to test for a current drain. Some computers can take up to 20 minutes to go to "sleep mode" and will draw up to three amps until then. The test light presents too much resistance. It limits current way too much to allow the computers to shut down. You need to insert an amp meter instead but the procedure involves using a jumper wire to bypass it whenever you switch scales to get more accuracy. The meter's switch breaks the circuit just long enough to wake the computers up again and that can draw enough current to blow the meter's internal fuse which is usually a 2 amp.

If the battery is self-discharging out of the car, which sounds like you've already identified the problem, I would charge it up, have it tested again, then leave it at Auto Zone for two or three days and have them test it again after it has been sitting. Be sure the top is clean and there's no condensed acid on it. That will cause self-discharge too.

Forgot to mention that since GM redesigned their generators for the '87 model year they have had a huge problem with them developing voltage spikes that destroy the internal diodes and voltage regulator as well as interfere with computer sensor signals. The battery dampens and absorbs those spikes but it loses its ability to do that as it ages. If the battery is more than about two years old it should be replaced to reduce the number of repeat generator failures.