selftests: tc-testing: reduce rshift value

After previous change rshift >= 32 is no longer allowed.
Modify the test to use 31, the test doesn't seem to send
any traffic so the exact value shouldn't matter.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250103182458.1213486-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>