I don't think there's enough evidence to support it either way, unfortunately. Their pre-Gear lore is just one step above being a bullet pointed list... Until Strive, even in Japanese, they used masculine pronouns to refer to them though. It's possible Testament could have been questioning before then or that Daisuke viewed them as nonbinary but just didn't know the terminology for it until more recently. Something that supports this is that Testament was included in both the male and female character polls in the back of the ASW 25th Anniversary Official Character Collection book (published Oct. 24, 2013).
They placed 23rd in the female character poll between Es and Kokonoe from Blazblue:
And 33rd on the male character poll between Potemkin and Carl Clover (Blazblue):
Daisuke's caption on one of the Testament illustrations in the Artworks of GGX 2000-2007 art book also suggests nonbinary/agender/genderfluid identities might have been something Daisuke was trying to place but possibly didn't know already existed or what terminology to use. I don't have the original Japanese text for this digitized, unfortunately, because I worked on this book before I started keeping records of the source text... Here's the translation I did of it though:
Testament is a character I would like to portray androgynously in a more honest way than Bridget. I wanted to make this character more feminine. Although my drawing skills are not up to the task, he has a beauty that women would be envious of. So, Testament’s design should lean more towards “beautiful” than “effeminate.” I would like to emphasize this inhuman beauty in future illustrations.
The "he" here was 彼 (kare) which can be used in a gender neutral way, but not in this context, especially since masculine pronouns had been used for Testament outside of this caption in the same—and previous—time periods.
That said, Testament is without a doubt nonbinary now and I think this intentional "vagueness" surrounding their gender going all the way back to Missing Link (in 1998!) is really interesting. If we had been given a piece of (canon) side media that gave Testament more "downtime," it makes me wonder if their previous gender identity would have been explored in a more concrete way than these hints given in polls and art book captions.