Trac #25217: Fix test regression in banner()

Fixes regression introduced in #25056 that assumes the tests are running
without the `SAGE_BANNER` environment variable set to a non-default.

(We should have a better mechanism in general for setting `sage.env` to
the default for tests that depend on it)

URL: https://trac.sagemath.org/25217
Reported by: embray
Ticket author(s): Erik Bray
Reviewer(s): Volker Braun

Author: Release Manager

src/sage/misc/banner.py

@@ -91,6 +91,7 @@ def banner():
 
     EXAMPLES::
 
+        sage: import sage.misc.banner; sage.misc.banner.SAGE_BANNER = ''
         sage: banner()
         ┌────────────────────────────────────────────────────────────────────┐
         │ SageMath version ..., Release Date: ...                            │