add basic unit tests to mixer plugin (#9479)
add basic unit tests to mixer plugin (#9479)

* add basic unit tests to mixer plugin

* minor lint

Signed-off-by: Rama <40109507d7c707c570c476fc1522e2d26df7f15e@salesforce.com>

smawson/istio,istio/istio,geeknoid/istio,smawson/istio,smawson/istio,geeknoid/istio,istio/istio,geeknoid/istio,istio/istio,geeknoid/istio,smawson/istio,geeknoid/istio,smawson/istio,istio/istio,geeknoid/istio,istio/istio,smawson/istio