In module mode, building the current directory requires a go.mod file
(in order to determine the import path of the package).

Change the tests to pass explicit file arguments instead, since those
can be built in module mode without defining a module.

Updates #30228

Change-Id: I680c658d1f79645f73ad4d1e88189ea50a4852e9
Reviewed-on: https://go-review.googlesource.com/c/162837
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>