Testing, at least at amateur level, is much easier for fixers than for developers. Measuring speed and grain (as Mark Overton did) requires some apparatus, and measuring sharpness (reliably!) is beyond reach for most of us. Measuring clearing speed is trivial, and tests for residual silver and hypo are cheap and available, with published recipes. The biggest obstacles to fixer improvements at the amateur level are the cheap availability of commercial rapid fixer (most of the time these commercial soups cost half or less of what the raw chems would cost us!) and the fact that fixers don't visibly improve your prints (for the first 10 years at least).
Is there a motivation for home brewing or testing fixers? Apparently TF-5 fixer runs circles around standard commercial rapid fixers, especially in terms of working solution life, but it's economically unfeasible to ship it across continents and its recipe is unpublished. A version of Farmer's reducer would be nice that has a working solution life of 8 hours (one really long dark room session). Likewise there is little to no data available for alkaline fixers, or even just simple data points like "what's the most economical way of archival fixing for TMX/TMY that leaves no magenta stain?" or "how can I translate a fixer's clearing time of film X into archival fixing time of paper Y?".