The develop run of these tests where logging:
go build github.com/juju/juju/vendor/github.com/oracle/oci-go-sdk/core:
D:\tools\Go\pkg\tool\windows_amd64\compile.exe: fork/exec
D:\tools\Go\pkg\tool\windows_amd64\compile.exe: The filename or extension is too long.
Installed Go 1.11 on Windows box running the tests on 8 Sep 2018, resolved issue