Generated test main packages have import paths that end with .test. Their package names are main. They should have one entry in GoFiles, which is inside the build cache.

{
	"Dir": "/Users/jayconrod/Code/test",
	"ImportPath": "m.test",
	"Name": "main",
	"Root": "/Users/jayconrod/Code/test",
	"Stale": true,
	"StaleReason": "stale dependency: m",
	"GoFiles": [
		"/Users/jayconrod/Library/Caches/go-build/4e/4ed90dfb73b1b23fb2030ce6dc74671fc821753d1018c3a52b818077266d5810-d"
	],
...
}

Is checking for those 3 things sufficient, or should we also check that the dependencies are exactly os, testing, and testing/internal/testenv?

Testing those 3 things should be sufficient. Even just the first two would be fine unless someone deliberately names their package something.test.

The list of imports is not stable. testing/internal/testenv could change. Also the -coverpkg flag will cause the test main to import every instrumented package, whether it's being tested or not.

Change https://golang.org/cl/213660 mentions this issue: internal/lsp: store workspace package IDs with package paths