I equally cannot reproduce this on go version devel +0f559941fb Sat Oct 26 15:17:28 2019 +0000 linux/amd64. I tried inside GOROOT/src and with various GO111MODULE values. I wonder if I missed any piece of the puzzle.
Reproduced with the instructions from the last comment.
It looks like go test -timeoout fmt is being interpreted as go test . with some arguments to pass to the tests, since -timeoout is not a flag that go test itself recognizes. So it tries to load the package in the current directory, $GOROOT/src.
The bug is that modload.ImportPaths is returning an empty string as one of the import paths. Which is kind of correct because normally a package path in std is its subdirectory under $GOROOT/src, and since there is no subdirectory in this case, we get "".
Maybe we should detect and reject this case specifically? It's such a small corner case though.