Remove or replace heka_plugin_raven

[say-yawn]

About this Issue

After accomplishing #150 and have moved from raven to sentry-sdk, some of the tests files uses the heck_plugin_raven which should be either be removed or replaced.

As @jwhitlock pointed out here

There are a few instances of raven in the shavar/tests folder, in config sections such as [heka_plugin_raven]. Those may need to be removed, or it could signal a new heka plugin is needed.

There's also a line in shaver.testing.ini, which is mentioned in README.md for local development, so that may be a place to figure it out.