I have not been able to find a simple MIDI test synth either. So, I made one using Max. It is very, very simple, made for testing only. It may do want you want. You can find it here:

Mitch

On Jun 18, 2020, at 4:25 AM, pd-list-request@lists.iem.at wrote:

Message: 4
Date: Thu, 18 Jun 2020 10:26:05 +0200
From: "Peter P." <peterparker@fastmail.com>
To: pd-list <pd-list@lists.iem.at>
Subject: [PD] Quicktime Synth via IAC Bus from Pd?
Message-ID: <20200618082605.btuqo77yycekwuip@fastmail.com>
Content-Type: text/plain; charset=us-ascii

Hi list,

when I still had to use OS X there was the Quicktime synthesizer
available as a Midi software synthesizer in the OS.
It was quite handy to quickly play a few notes from Pd's Midi objects.
It seems to be gone from current (mobile-phone-like) versions of OS X. Is
this correct? Is there an alternative inside the OS or can anyone
recommend a good (preferable open source) replacement that works over
the IAC bus?

Thanks!
P