JSON output flag not implemented for amtool config routes test

What did you do?

Tried to format the output from amtool config routes test as JSON using the --output=json flag.

What did you expect to see?

The list of matched routes as a JSON list, instead of the comma separated strings, e.g.

# amtool config routes test --output=json --config.file=alertmanager-config.yaml --verify.receivers=Default,slack-default namespace=example alertname=KubePodNotReady              
["Default", "slack-default"]

What did you see instead? Under which circumstances?

The default comma separated string where being shown, e.g.

# amtool config routes test --output=json --config.file=alertmanager-config.yaml --verify.receivers=Default,slack-default namespace=example alertname=KubePodNotReady              
Default,slack-default

Environment

• System information:

Linux 6.7.6-arch1-1 x86_64

• Alertmanager version:

alertmanager, version 0.26.0 (branch: tarball, revision: 0.26.0)
  build user:       someone@builder
  build date:       20230825-18:04:14
  go version:       go1.21.0
  platform:         linux/amd64
  tags:             unknown

• Prometheus version:

not relevant for this issue

• Alertmanager configuration file:

not relevant for this issue

• Prometheus configuration file:

not relevant for this issue

• Logs:

not relevant for this issue

Start by locating the amtool config routes test command and the existing --output handling for route results. Compare its behavior with other JSON output paths, then verify that the command emits a JSON list of matched receivers rather than comma-separated text.