Combine models of JH and Upstream for Import/Export test

Why need it

Combine the models of JH and Upstream together.

# Part of the test security suite for the Import/Export feature
# Finds if a new model has been added that can potentially be part of the Import/Export
# If it finds a new model, it will show a +failure_message+ with the options available.

ALl Upstream models are listed in spec/lib/gitlab/import_export/all_models.yml.

We need to let the test know about the newly added JH only model.

What does this MR do

• Override test variable all_models_hash to combine the models of JH and Upstream together
• Upstream prompt text Definitely add it to #{File.expand_path(jh_models_yml)}`

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cc EM @prajnamas