For example, if a test calls os.Exit(0), that could trick a 'go test'
run into not running some of the other tests, and thinking that they all
succeeded. This can easily go unnoticed and cause developers headaches.

Add a simple sanity check as part of 'go test': if the test binary
succeeds and doesn't print anything, we should error, as something
clearly went very wrong.

This is done by inspecting each of the stdout writes from the spawned
process, since we don't want to read the entirety of the output into a
buffer. We need to introduce a "buffered" bool var, as there's now an
io.Writer layer between cmd.Stdout and &buf.

A few TestMain funcs in the standard library needed fixing, as they
returned without printing anything as a means to skip testing the entire
package. For that purpose add testenv.MainMust, which prints a warning
and prints SKIP, similar to when -run matches no tests.

Finally, add tests for both os.Exit(0) and os.Exit(1), both as part of
TestMain and as part of a single test, and test that the various stdout
modes still do the right thing.

Fixes #29062.

Change-Id: Ic6f8ef3387dfc64e4cd3e8f903d7ca5f5f38d397
Reviewed-on: https://go-review.googlesource.com/c/go/+/184457
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>