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

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

Also, while here

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

Reviewed by: tobik
Differential Revision: https://reviews.freebsd.org/D22412