Write the specs properly

What does this MR do and why?

I have noticed some specs were not written properly.

• ee/spec/lib/elastic/latest/git_class_proxy_spec.rb: Send one more project_id in the options[:project_ids] to test the multiple routing feature. I found that it was not working so I had to change the stubbing strategy in the file ee/spec/support/helpers/elasticsearch_helpers.rb

• ee/spec/lib/gitlab/elastic/search_results_spec.rb: project_id will always be present in the _source so added this field also in the _source for the test. To give the context why I fixed this because in the case of GroupWiki project_id will be missing and that's how I will differentiate the Group level search from the project-level search

• ee/spec/services/search/group_service_spec.rb: It is a group search, so project should be under a group. We were setting group as project.namespace but again the issue is project.namespace is a UserNamespace. Similar case as the first one.

• ee/spec/support/helpers/elasticsearch_helpers.rb: When there is multiple routing like routing=project_1,project_2, the comma(,) was getting encoded in %2C and the stubbing was not working, so I have used the regex to create URI.

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