update-test: more improvements.

[MikeMcQuaid]

• Explain why HOMEBREW_UPDATE_TEST is set and what it does.
• Avoid auto-updating (this should already be not happening but let's make sure).
• Set environment variables to ensure we're testing the default update behaviour (rather than deferring to user configuration).
• Use HOMEBREW_BREW_GIT_REMOTE as well as setting git config remote.origin.url ourselves.
• Run brew help quietly first to hide irrelevant portable Ruby output.
• Run brew update --verbose --debug to get even more output on failure.

Fixes #8979

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

---