Add e2e test of SSH pull mirroring with a private key

What does this MR do?

Adds an e2e test of SSH pull mirroring with a private key.

• Adds new QA selectors and updates a few to use data attributes.
• Replaces a qa- selector used in a unit test with an rspec- selector.
• Changes select_element to not capitalize the input. Updates tests affected by the change.
• Changes mirror direction and auth method selection to use strings rather than symbols (which could not easily allow "SSH public key" to be selected if specified as a symbol).
• Adds assert_no_element that uses Capybara's assert_no_selector, which will raise an error if the selector exists.

Closes gitlab-org/quality/testcases#228 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content