[ResponseOps] add function tests for action mustache variables

[pmuellr]

While reviewing PR #147136, I realized that we didn't have any function tests for action mustache variables! As near as I could tell. I think we should. For the common variables shared across all rules - so everything except the context.* variables.

Basic idea would be to create any rule, add an index action (or some other action we can capture the output from), get the rule to trigger, then check that the action was executed with the expected variable values.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)