(maint) Add test for invalid PQL queries on entity endpoints
(maint) Add test for invalid PQL queries on entity endpoints

PQL queries on query entity endpoints (like query/v4/inventory) should
return a 400 HTTP status and produce an error message about a failed
JSON parse attempt.

puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb