With one exception, the go command takes arguments in the form of packages.

You can pass the package name(s) explicitly, eg.

go test github.com/hoisie/mustache

or implicitly

cd $GOPATH/src/github.com/hoisie/mustache
go test .

By default, if no arguments are provided go test treats the current directory as a package, so in the second example, go test . is identical in function to

cd $GOPATH/src/github.com/hoisie/mustache
go test

The ability to pass a single file to go {build,test,install} is a degenerate case brought about by go run accepting a single .go file as its argument.

go run should be considered the single exception to the rule outlined above.