selftests/tc-testing: Adapt tc police action tests for Gb rounding changes

selftests/tc-testing: Adapt tc police action tests for Gb rounding changes

For the tc police action, iproute2 rounds up mtu and burst sizes to a
higher order representation. For example, if the user specifies the default
mtu for a police action instance (4294967295 bytes), iproute2 will output
it as 4096Mb when this action instance is dumped. After Jay's changes [1],
iproute2 will round up to Gb, so 4096Mb becomes 4Gb. With that in mind,
fix police's tc test output so that it works both with the current
iproute2 version and Jay's.

[1] https://lore.kernel.org/netdev/20250907014216.2691844-1-jay.vosburgh@canonical.com/

Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Reviewed-by: Cong Wang <cwang@multikernel.io>
Reviewed-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Link: https://patch.msgid.link/20250912154616.67489-1-victor@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>