tests: allow custom realm notice on stderr

[ffumero2003]

Summary

• allow the intended custom-realm Using http(s)://… notice in integration-test stderr
• keep every other stderr line failing
• link the rule to the matching unit-test expectation (test/unit/test_console_tool.py)

When B2_ENVIRONMENT is a custom URL, the CLI prints a Using <url> notice to stderr; the integration suite's shared should_succeed asserted clean stderr, so every command erroring on that notice (55 errors when the suite is retargeted to a simulator). This narrows the allow-list to that one notice, mirroring the regex the unit tests already use. No CLI runtime behavior changes.

Verification

• focused helper check: the custom-URL notice is accepted; unrelated stderr still fails
• simulator integration run: the Unexpected stderr line: 'Using http://…' errors are gone

Note

Same change as #1180 (which merged into development); this is the master-targeted PR.