go/build: clean up ctxt.shouldBuild, tests

Make ctxt.shouldBuild return multiple values
instead of modifying *sawBinaryOnly in place.
Also give it a table-driven test.

Cleanup in preparation for boolean expressions,
but nice even if those don't end up happening.

Change-Id: Ibb78b0080070deafac7299a6de87ab8bebeb702d
Reviewed-on: https://go-review.googlesource.com/c/go/+/240598


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>