selftests: fib_rule_tests: Add VRF tests

selftests: fib_rule_tests: Add VRF tests

After commit 40867d74c374 ("net: Add l3mdev index to flow struct and
avoid oif reset for port devices") it is possible to configure FIB rules
that match on iif / oif being a l3mdev port. It was not possible before
as these parameters were reset to the ifindex of the l3mdev device
itself prior to the FIB rules lookup.

Add tests that cover this functionality as it does not seem to be
covered by existing ones and I am aware of at least one user that needs
this functionality in addition to the one mentioned in [1].

Reuse the existing FIB rules tests by simply configuring a VRF prior to
the test and removing it afterwards. Differentiate the output of the
non-VRF tests from the VRF tests by appending "(VRF)" to the test name
if a l3mdev FIB rule is present.

Verified that these tests do fail on kernel 5.15.y which does not
include the previously mentioned commit:

 # ./fib_rule_tests.sh -t fib_rule6_vrf
 [...]
     TEST: rule6 check: oif redirect to table (VRF)                      [FAIL]
 [...]
     TEST: rule6 check: iif redirect to table (VRF)                      [FAIL]

 # ./fib_rule_tests.sh -t fib_rule4_vrf
 [...]
     TEST: rule4 check: oif redirect to table (VRF)                      [FAIL]
 [...]
     TEST: rule4 check: iif redirect to table (VRF)                      [FAIL]

[1] https://lore.kernel.org/netdev/20200922131122.GB1601@ICIPI.localdomain/

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240409110816.2508498-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>