Attempt to fix a problem with E2E tests

[gudmdharalds]

This pull request attempts to resolve a problem sometimes encountered when running E2E tests (probably when not on a pull request branch, see example). With the added reference, there should hopefully always be a branch available when checking out.

TODO:

• Added patch for E2E test checkout
• [N/A] Add/update tests -- unit/integrated/E2E (if needed)
  • [N/A] Ensure only one function/functionality is tested per test file.
• [N/A] Add to, or update, Scan run detail report as applicable
• Check status of automated tests
• [N/A] Ensure PHPDoc comments are up to date for functions added or altered
• Assign appropriate priority and type of change labels.
• Changelog entry (for VIP) [ Add changelog for version 1.3.9 #377 ]

[wpcomvip-vipgoci-bot]

Auto-approved pull request #376 as it contains only auto-approvable files -- non-functional changes to PHP files or file-types that are auto-approvable (css, csv, eot, gif, gz, ico, ini, jpeg, jpg, json, less, map, md, mdown, mo, mp4, otf, pcss, pdf, po, pot, png, sass, scss, styl, ttf, txt, woff, woff2, yml).