amtool: add support for stdin to `config routes test`

[MPV]

What did you do?

$ command-that-outputs-generated-alertmanager-config | docker run --rm \
        --entrypoint amtool --name alertmanager -i \
        -v $PWD:/work -w /work \
        quay.io/prometheus/alertmanager \
        config routes test alertname="foo", severity="critical"

What did you expect to see?

Validation of my routes passed via stdin.

Similar to what was added support for in #1431

What did you see instead? Under which circumstances?

amtool: error: You have to specify one of --config.file or --alertmanager.url flags.

I also tried passing - (common identifier for stdin) as --config.file=-

[sinkingpoint]

If you're on a linux system you can use the /dev/stdin device e.g.

echo <config> | docker run --rm \
        --entrypoint amtool --name alertmanager -i \
        -v $PWD:/work -w /work \
        quay.io/prometheus/alertmanager \
        config routes test --config.file /dev/stdin alertname="foo", severity="critical"

But I agree - support for - would be more consistent