Add GO_TEST_TARGET defaulting to `./...` (the current package and all
subpackages) and create do-test target unless already provided by port's
In many cases this would allow us to remove explicit do-test and rely on
defaults provided by ports framework for testing.
While here also
- remove GO_WRKDIR_SRC - it is not used anywhere anymore
- sync GO_PKGNAME and GO_TARGET descriptions with Porter's Handbook