bpo-45400: Fix suggestion test of test_exceptions (GH-28783)

Fix test_name_error_suggestions_do_not_trigger_for_too_many_locals()
of test_exceptions if a directory name contains "a1" (like
"Python-3.11.0a1"): use a stricter regular expression.
(cherry picked from commit 4e605666b08b8f863cbbbdaa34bb06988e648d26)

Co-authored-by: Victor Stinner <vstinner@python.org>