Disable new tab background for tests

[cezaraugusto]

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Auditors: @bsclifton
Fix: #9240

Most tests were created while new tab was a blank page and/or before we had the option to show gradient background.

Test Plan:

1. While running tests, new tab background should be shown as background gradient rather than the background image.
2. No test regressions should happen

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[NejcZdovc]

I would go other way around. So I think that non of the tests need background so I would disable it on global level, so if NODE_ENV test set this value to false or when you initialize test in lib/brave. This way tests are lighter and you can enable if needed.

[luixxiul]

Isn't documentation needed on docs/ to let people know that the images are disabled by default and they need to set something to run tests that include dashboard images?

[luixxiul]

++ on docs update :-)

[NejcZdovc]

@cezaraugusto can you please squash commits

[bsclifton]

Great change; this will help save testing time! 😄

[bsclifton]

@NejcZdovc can you please re-review (and merge if it looks OK 😄 )

[NejcZdovc]

LGTM ++