"make test" fails

[dkegel-fastly]

With go 1.16.3, doing
make test
fails with

go install github.com/pquerna/ffjson
go install: version is required when current directory is not in a module
	Try 'go install github.com/pquerna/ffjson@latest' to install the latest version

Following the suggestion does not work; it seems to produce the same error.

Working around this by creating a trivial go.mod, then doing go mod tidy, fails with

	github.com/foo/vendored: cannot find module providing package github.com/foo/vendored: module github.com/foo/vendored: git ls-remote -q origin in /Users/dkegel/go/pkg/mod/cache/vcs/d5727a80ed1da50c9aefa6067dac442a4ec8c5b1a6b78d2164bed13a43544b3d: exit status 128:

which is rather like a modern version of #239