Elasticsearch Delete by query before examples

What does this MR do?

Related to #325719 (closed)

Add a new Rspec test helper that only creates/deletes the indexes at the start of the context. The Elasticsearch API DeleteByQuery is used to delete all data in Elasticsearch before each example instead. This is done by using delete_by_query with a match_all query (with 5 slices, and instructing ES to proceed on conflicts)

Before

• ee/spec/services/search/global_service_spec.rb: 2188.1423042750002
• ee/spec/services/search/group_service_spec.rb: 2122.4184179000003
• ee/spec/services/search/project_service_spec.rb: 1955.100622246
• ee/spec/services/search/snippet_service_spec.rb: 337.0130661719995

After

• ee/spec/services/search/global_service_spec.rb: 913.0082140049999
• ee/spec/services/search/group_service_spec.rb: 985.0923684649988
• ee/spec/services/search/project_service_spec.rb: 956.9252228980004
• ee/spec/services/search/snippet_service_spec.rb: 83.09271206999983

Screenshots (strongly suggested)

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