Normally to test Alertmanager templates you need to restart running Alertmanager and wait for alerts to arrive to Slack or email. To speed up this process parts of templates could be rendered locally using predefined alerts data without the need of the actual Alertmanager.
What you still need in this case is amtool which is a part of Alertmanager delivery which could be downloaded here https://github.com/prometheus/alertmanager/releases
Some examples of templates rendering:
# navigate to the templates dir