cmd/go: always build _test.go files and link into test
go test's handling of _test.go files when the entire
package's set of files has no Test functions has varied
over the past few releases. There are a few interesting
cases (all contain no Test functions):
(1) x_test.go has syntax errors
(2) x_test.go has type errors
(3) x_test.go has runtime errors (say, a func init that panics)
In Go 1.1, tests with (1) or (2) failed; (3) passed.
In Go 1.2, tests with (1) or (2) failed; (3) passed.
In Go 1.3, tests with (1) failed; (2) or (3) passed.
After this CL, tests with (1), (2), or (3) all fail.
This is clearly a corner case, but it seems to me that
the behavior of the test should not change if you
add or remove a line like
func TestAlwaysPasses(t *testing.T) {}
That implies that the _test.go files must always
be built and always be imported into the test binary.
Doing so means that (1), (2), and (3) must all fail.
Fixes #8337.
LGTM=iant
R=golang-codereviews, iant
CC=adg, golang-codereviews, r
https://golang.org/cl/
150980043