support reading stdout/stderr from streams

[pohly]

It can be useful to let the user run the test command(s) itself, for example when complex shell scripts are involved which need to invoke go test multiple times.

With -stdin, the go test stdout is expected on stdin of gotestsum, so it can be used in a pipe. To detect abnormal termination of the test commands, bash with "set -o pipefail" should be used. Beware that such failures are not detected by gotestsum.

To also capture stderr with gotestsum, stderr must get redirected like this:

mkfifo /tmp/pipe
go test ... 2>/tmp/pipe | gotestsum -stdin -stderr 3 3</tmp/pipe

Fixes: https://github.com/gotestyourself/gotestsum/issues/414

[pohly]

I added documentation and unit tests. I also checked that this really works for Kubernetes: https://github.com/kubernetes/kubernetes/pull/125534.