Bug in coverage loading functions breaking the CI build if enabled

After the merge of #3404 that uses a common class for pack and unpack, the coverage loader is broken as it basically load all files without preference.

see:

```
module CoverageHelper
def self.eager_load
Dir.glob("{logstash-core{/,-event/},}lib/**/*.rb") do |file|
next if file =~ SKIP_LIST
require file
end
end
end
```

so for the pack/unpack command this break when the common class is not laded before them. This breaks the build as you can see in http://build-eu-00.elastic.co/job/logstash_regression_master/493

For now coverage reports are disabled for regression test, should be re enabled as soon as this is fixed.