Prepare Elastic indices only before context, not before examples

What does this MR do?

In some Elastic-related tests, we don't have to rebuild the indices before each test, and delete them after each test, we can just create the indices in a before(:context) block and delete them in an after(:context) block.

By doing that, we easily reduce the timing of ee/spec/services/search/project_service_spec.rb by half, from 29 minutes to 14 minutes. 💥

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because test.
• Documentation (if required)
• Code review guidelines
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• Database guides
• Separation of EE specific content

Related to #325719 (closed).