use CHROME_USER_DATA_DIR to set/override the user data dir in tests

[bridiver]

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.

Test Plan:
messages about deleting directories should mostly go away in automated test runs
BRAVE_USER_DATA_DIR can be used to override the directory for tests

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

[ayumi]

related muon PR: brave/muon#375

[bridiver]

muon PR is related, but not required. It's broken already and this doesn't make it any more broken