I'm looking for the source code of the "go test" command which afaik does some funky stuff (exports the functions, recompiles the package etc). So far I could find only "testing" and the cmd/go doc
You can start looking in the cmd/go package, where most of the go commands are defined:
https://github.com/golang/go/blob/1ba29926f3bfd245d46cf1f287716290bc2f1034/src/cmd/go/test.go (last known commit with src/cmd/go/test.go, Go 1.8)
Then commit 4efe925 shows src/cmd/go/test.go being renamed/moved to src/cmd/go/internal/test/test.go in Go 1.9, Jan. 2017.
Last modification: commit ca33f33 in Oct. 2018 for Go 1.12.