test: remove dependency on source code in GOPATH

[hyangah]

Current tests require GOPATH env var to be explicitly set
(

vscode-go/test/integration/extension.test.ts

Line 43 in 341a4ff

assert.ok(gopath, 'Cannot run tests if GOPATH is not set as environment variable');

)

and depend on source code preexisting under GOPATH/src.
(

vscode-go/test/integration/extension.test.ts

Line 663 in 341a4ff

// Since the Go extension takes a dependency on the godef tool at github.com/rogpeppe/godef

)

That is not ideal. Fix that.

[gopherbot]

Change https://golang.org/cl/244771 mentions this issue: test/integration: do not depend on real packages in testing

[gopherbot]

Change https://golang.org/cl/244772 mentions this issue: build,.github/workflows: install all tools in modules mode