Refactor External-link component test file to new format

[rmccar]

What is the context of this PR?

Fixes #153 in the design team issues board

Refactors the External-link component test file to the new agreed format

How to review this PR

Run tests locally and check that the messages make sense and describe what is being tested

Ask yourself....

• Do we need any additional tests?
• Are the tests descriptions clear?
• Does the order of the test document make sense?
• Is it internally consistent?
• Is it consistent with the guidelines for test refactoring?
• run this test command to view the test results:
• yarn test --testNamePattern="FOR: Macro: External-link"

Checklist

• [x] I have selected the correct Assignee
• [x] I have linked the correct Issue

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