goapp test not working,getting error "GOPATH must be absolute" while it is absolute

I'm tying to write tests for my google cloud app. I read the documents and it seems the only way to run the test locally is the running the command goapp test in the test package directory. But when I run the command I get the error go: GOPATH entry is relative; must be absolute path: "".

I'm pretty sure my GOPATH is set absolutely. Here are the results when I run the command go env | grep GOPATH:

GOPATH=":/home/mohammad/go:/home/mohammad/go/src/gitlab.com/gc-auth"

Also getting the same output when I run echo $GOPATH.

Any help is appreciated.

PS: I have ubuntu 18.04 and my go version is 1.10.4

results of gcloud version:

Google Cloud SDK 228.0.0 app-engine-go app-engine-python 1.9.80 bq 2.0.39 cloud-datastore-emulator 2.0.2 core 2018.12.07 gsutil 4.34