Famously, Bobby Fischer describes the first move 1. e4
as "best by test". However,
Bobby Fischer exclusively opened his games with 1.e4...
“Best by test: 1.e4” (Bobby Fischer)
Actually, Fischer didn't play 1.e4 100% exclusively, but it's very close. So it seems like Bobby Fischer didn't actually test non-e4 openings during tournaments all that much. As black, he scored better against 1.e4 than almost any other opening.
Hence the question:
Question: What test is Fischer referring to when describing 1.e4 as "best by test"?