Mk/Uses/go.mk: Add GO_TEST_TARGET and provide default do-test

Add GO_TEST_TARGET defaulting to ./... (the current package and all
subpackages) and create do-test target unless already provided by port's
Makefile.

In many cases this would allow us to remove explicit do-test and rely on
defaults provided by ports framework for testing.

Also, while here

• remove GO_WRKDIR_SRC - it is not used anywhere anymore
• sync GO_PKGNAME and GO_TARGET descriptions with Porter's Handbook

Reviewed by: tobik
Approved by: tz (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D22412