Convert `Rails.env` stub usages to use `RailsHelpers.stub_rails_env` instead

Context: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15715#note_211720503

$ git grep "allow(Rails.env)" spec
spec/features/help_pages_spec.rb:      allow(Rails.env).to receive(:production?).and_return(true)
spec/features/user_can_display_performance_bar_spec.rb:      allow(Rails.env).to receive(:development?).and_return(true)
spec/helpers/icons_helper_spec.rb:        allow(Rails.env).to receive(:development?).and_return(true)
spec/helpers/icons_helper_spec.rb:        allow(Rails.env).to receive(:test?).and_return(true)
spec/helpers/icons_helper_spec.rb:        allow(Rails.env).to receive(:test?).and_return(false)
spec/helpers/icons_helper_spec.rb:        allow(Rails.env).to receive(:development?).and_return(false)
spec/helpers/version_check_helper_spec.rb:      allow(Rails.env).to receive(:production?) { false }
spec/helpers/version_check_helper_spec.rb:        allow(Rails.env).to receive(:production?) { true }
spec/lib/gitlab/database/migration_helpers_spec.rb:      allow(Rails.env)
spec/lib/gitlab/database/migration_helpers_spec.rb:      allow(Rails.env)
spec/lib/gitlab/database/migration_helpers_spec.rb:      allow(Rails.env)
spec/lib/gitlab/database/migration_helpers_spec.rb:      allow(Rails.env)
spec/lib/gitlab/database/migration_helpers_spec.rb:      allow(Rails.env)
spec/lib/gitlab/gitaly_client_spec.rb:        allow(Rails.env).to receive(:production?).and_return(true)
spec/lib/gitlab/query_limiting_spec.rb:      allow(Rails.env).to receive(:development?).and_return(true)
spec/models/concerns/sha_attribute_spec.rb:        allow(Rails.env).to receive(:production?).and_return(false)
spec/models/concerns/sha_attribute_spec.rb:        allow(Rails.env).to receive(:production?).and_return(true)
spec/models/internal_id_spec.rb:        allow(Rails.env).to receive(:test?).and_return(false)
spec/tasks/gitlab/gitaly_rake_spec.rb:        allow(Rails.env).to receive(:test?).and_return(false)
spec/tasks/gitlab/gitaly_rake_spec.rb:            allow(Rails.env).to receive(:test?).and_return(true)