Bug 16453: Make Elasticsearch tests be skipped if configuration entry missing

Bug 16453: Make Elasticsearch tests be skipped if configuration entry missing

The current tests fail to run if the configuration entry is missing. This is
problematic on jenkins and should be fixed.

To test:
- On master, having koha-conf.xml without (or commented) an <elasticsearch> entry
- Run:
  $ prove t/db_dependent/Koha_ElasticSearch_Indexer.t \
          t/db_dependent/Koha_SearchEngine_Elasticsearch_Search.t -v
=> FAIL: Tests fail due to missing configuration entry
- Apply the patch
- Run:
  $ prove t/db_dependent/Koha_ElasticSearch_Indexer.t \
          t/db_dependent/Koha_SearchEngine_Elasticsearch_Search.t -v
=> SUCCESS: Tests pass, and the ES-configuration-dependent tests are skipped
- Have elasticsearch running and the koha-conf.xml entry
- Run:
  $ prove t/db_dependent/Koha_ElasticSearch_Indexer.t \
          t/db_dependent/Koha_SearchEngine_Elasticsearch_Search.t -v
=> SUCCESS: Same results as without the patch
- Sign off

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Jesse Weaver <jweaver@bywatersolutions.com>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>