logstash: Restore test of config file contents
logstash: Restore test of config file contents

Commit 7419181 changed TestGetPipelineConfigDir() to no longer check
that the copied configuration file contained the expected contents.
Restore the old behavior, but since the config file is actually
parsed the configuration file given to the code under test must
actually be valid so we use the github.com/breml/logstash-config/ast
package for that.

magnusbaeck/logstash-filter-verifier