A CG Products PCV3 and a Mazzatron Testy allows you to test ANY synth patchcable...1/4", banana, and 3.5mm.
The Animodule TrAniModule gives you a 1 watt output amplifier and speaker to check audio outs.
The Joranalogue TEST 3 lets you check individual module current draws and other important DC level checks.
A 2hp DC gives you three possible fixed voltages...very useful for checking CV scaling and tracking.
The big draw here, of course, is Jones's O'Tool's latest revision, offering frequency measurement, an o-scope, voltmeter, and a pile of other test functions.
And last, a Metro Modular Test Eagle Option 10 gives you four dual bananas for connecting external test gear, with appropriate impedance and level settings for typical devices of that sort.
This isn't a synthesizer, of course...but if you have a sizable Eurorack (or other) modular rig, a little skiff like this with these sorts of modules definitely gives you 95% of the key test functions you need to keep that synth operating to spec.