gh-91821: Make decimal test succeed consistently (#91825)

The test relies on precision being set to 9, but some ways of
invoking this test leave it set to 28 instead. I don't know
exactly how it happens, but setting the precision directly should
make the behavior consistent.

Lib/test/test_decimal.py

@@ -5520,6 +5520,7 @@ class CWhitebox(unittest.TestCase):
 
         with localcontext() as c:
 
+            c.prec = 9
             c.traps[InvalidOperation] = True
             c.traps[Overflow] = True
             c.traps[Underflow] = True