Add RTL test to @shared/GithubBuildShaLink.jsx

[apburnes]

Add a test to @shared/GithubBuildShaLink.jsx using the RTL framework. See our docs for writing frontend tests.

Acceptance Criteria

• Colocate test file to @shared/GithubBuildShaLink.test.jsx
• Test the GithubLink component renders with the build.clonedCommitSha prop
• Test the GithubLink component renders with the build.requestedCommitSha prop
• Test nothing renders when the build.requestedCommitSha and build.clonedCommitSha are not available
• Remove component defaultProps and add the to the component args