The docking adapters for Gemini and Agena were apparently tested for Gemini VI; [there's at least one photo, S65-52015, of the ground test](https://images.nasa.gov/details/S65-52015).  Gemini VI's Agena "[failed to go into orbit](https://www.nasa.gov/mission/gemini-vi/)" and the Gemini VI-A mission rendezvoused with Gemini VII instead.

[![GEMINI-TITAN (GT)-6 - AGENA TESTS - DOCKING EXERCISE - BORESIGHT RANGE - CAPE S65-52015 (1965) --- The Gemini-6 spacecraft (right) and the Agena Target Vehicle (left) on the Boresite Range Tower for the Plan-X docking exercise. Photo credit: NASA or National Aeronautics and Space Administration ][1]][1]

For Gemini 8 specifically, [NASA Press Kit for Friday AM March 11, 1966, Release No 66-52, Gemini 8 Launch Set for March 15](https://ntrs.nasa.gov/api/citations/19660026037/downloads/19660026037.pdf) tells us
> The Agena and its docking adapter were shipped to the Kennedy Center Jan. 21. These two components and the Gemini spacecraft were mounted atop a 50-foot "Timber Tower" at KSC'S Radio Frequency Test Systems site Jan. 27-28. The prime Gemini 8 crew and their backups boarded the spacecraft on the tower to conduct a series of Radio Frequency Capability Tests between Gemini 8 and the Agena target. **Docking compatibility checks also were made between the two vehicles.**

Emphasis mine.  It isn't clear to me if the "Boresight Range Tower" of the Gemini VI photo is the same as the "Timber Tower" referred to in the press release.

[On Astronautix](http://www.astronautix.com/g/geminiagenatargetvehicle.html) such tests appear to be referred to as "Combined Interface Tests", starting with Gemini 6. The same term is used in [the link to SP-4002 that Controlgroup shared](https://www.nasa.gov/history/SP-4002/p3b.htm).

All found by Googling "gemini agena interface test."

  [1]: https://i.sstatic.net/z1Smaj85.png