This looks weird. Are you using
go test or
go test <filename>? (See the last two comments in this related issue that observe the issue with
go test <filename> only.)
Does this only happen with
go test (versus
go build, etc)?
Did you check all of the involved files for backslashes (not only the one that occurs in the error message)?
In case you use an older Go version, it might be worth trying the latest one.
Also, if you have access to a Linux machine, verify if the issue replicates there. (Assuming you currently use a Windows machine.)