cmd/go: build errors rather than strings in some tests
Speed up the test suite by building the errors package rather than the
strings package in some cases where the specific package we are
building doesn't matter. The errors package is smaller, and doesn't
have any assembler code.
Also make a couple of tests run in parallel.
Update #11779.
Change-Id: I62e47f8655f9d85bf93c70ae6e6121276d96aee0
Reviewed-on: https://go-review.googlesource.com/12365
Run-TryBot: Ian Lance Taylor <iant@golang.org> Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>