Add test functionality for unsaved group integrations

What does this MR do and why?

Currently, group-level integrations that support being 'testable' (only Jira?), will always fall when pressing 'Test settings' because we don't actually make a callout to a test function. This MR uses the same functionality as the project-level integrations by adding #test to Groups::Settings::IntegrationsController, which in turn calls integration_test_response. There is already existing functionality in Integrations::Test::GroupService to handle the test.

Resolves #461000

MR acceptance checklist

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Screenshots or screen recordings

The following example should return a failure message that the URL cannot be resolved. The first test in Before is taken from GitLab.com. After is in GDK with these changes applied.

Before	After

How to set up and validate locally

1. Use an existing group, or create a new top-level group.
2. Go to Settings → Integrations, and select Jira.
3. Use either a valid Atlassian Jira server, or a fake URL, to confirm that the test now properly performs the callout.