Understanding go mod and cause of package is not in GOROOT

I'm trying to play around with the lightning network. I have cloned the repo, and on disk placed it here (I'm using windows):

C:\Users\hallibut\Documents\GitHub\lnd

I'd like to run any of the tests in itest, lets say testMultiHopPayments. The cli commands I'm using after I cd into the above location is:

go test itest -run testMultiHopPayments

However, I keep getting the error:

package itest is not in GOROOT (C:\Program Files\Go\src\itest)

I've read through the various posts on this error, but I'm still not quite sure why it happens, and it's likely because I don't fully understand the go module (I'm new to go). This article, was probably the best in helping me understand the structure and env variables:

https://golangbyexample.com/workspace-hello-world-golang/

My understanding from the various readings is that whatever directory the go.mod file is in, indicates the module level directory. Prior to version 1.13 there was a required directory and structure, but now that should not be an issue if you're using at least version 1.13 and modules. I'm using 1.17.1. This is somewhat of an assumption or inference, but I believe everything lower in the directory structure is part of a package to be installed as part of the module (and is defined by the package keyword). However, I don't understand why a package with source code within a subdirectory would be missing/throw the aforementioned error. I've also tried running:

go mod install github.com/lightningnetwork/lnd/lntest/itest

That doesn't seem to do anything/has not effect on the error. What am I not understanding about packages? Looking at the go.mod file I observe that itest is not specifically defined anywhere… Not sure if that's required. Also, I assume I've got to run some build process prior? I attempted this with:

go install -v ./...

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