Summary:
Our `testmaingen` has not been updated since go1.18 (or maybe even before that). It's super stale and out-of-date.

Update the file using the latest go1.24 branch:
https://github.com/golang/go/blob/release-branch.go1.24/src/cmd/go/internal/load/test.go

This will allow us to migrate to the new coverage redesign go1.20+ (downstream). While making intermediate diff reviews easier.

Reviewed By: podtserkovskiy

Differential Revision: D68396999

fbshipit-source-id: f12b184ce19413e50872a34a0427199053b99f68