I found out that if I run all.bash and build, then run the test with that newly built version the test passes, but for some reason running the test with the installed version it fails. They're the same versions, both 1.16.3. So I get different results depending on what directory the go command is in when I run the test.

Exported things in *_test.go files are only visible for the test package in the same directory. When using old Go for running new Go's bufio_test, new one can't refer exported things in old Go's bufio/export_test.go file, because the import statement import . "bufio" in new Go's bufio_test refers to old Go's bufio package in this situation.

I think there is no bug here.

OK thanks. Where is the package import search order specified?

There is no package import search order as such, because the package paths mentioned in import statements are unique (https://golang.org/cmd/go/#hdr-Package_lists_and_patterns).

All code in *_test.go files can only be seen when running go test for that package. They can not be imported by other packages.

Closing the issue because this is working as intended.

Thanks @ianlancetaylor. That page states: If no import paths are given, the action applies to the package in the current directory.
Maybe it should be amended to say: If no import paths are given, the action applies to the package in the current directory unless the current directory holds package source code for a std lib package, in which case the GOROOT is used for the action.