Add manticore header to unit tests to avoid transient error

[alexcams]

There was a CI transient error while executing elasticsearch_spec.rb unit tests that was making pipelines fail from time to time:
Uninitialized Manticore error

This is a concurrency error caused by the indirect inclusion of Manticore library while calling this function that is called by many tests in elasticsearch_spec.rb file. Since the tests execution does not always follow the same order, the problem comes if any test that use Manticore::Client class is executed before calling this function that indirectly includes the Manticore library, it causes this class to be "uninitialized".

To solve this transient error, I've just included the Manticore lib at the beginning of the file.

Thanks for contributing to Logstash! If you haven't already signed our CLA, here's a handy link: https://www.elastic.co/contributor-agreement/

[jsvd]

Tests are failing due to fixture certs expiring, please run spec/filters/fixtures/generate_test_certs.sh and commit the new certs to this PR

[jsvd]

LGTM