selftests: fib_rule_tests: Clarify test results

Clarify the test results by grouping the output of test cases belonging
to the same test under a common title. This is consistent with the
output of fib_tests.sh.

Before:

 # ./fib_rule_tests.sh

     TEST: rule6 check: oif redirect to table                            [ OK ]

     TEST: rule6 del by pref: oif redirect to table                      [ OK ]
 [...]
     TEST: rule4 check: oif redirect to table                            [ OK ]

     TEST: rule4 del by pref: oif redirect to table                      [ OK ]
 [...]

 Tests passed: 116
 Tests failed:   0

After:

 # ./fib_rule_tests.sh

 IPv6 FIB rule tests
     TEST: rule6 check: oif redirect to table                            [ OK ]
     TEST: rule6 del by pref: oif redirect to table                      [ OK ]
 [...]

 IPv4 FIB rule tests
     TEST: rule4 check: oif redirect to table                            [ OK ]
     TEST: rule4 del by pref: oif redirect to table                      [ OK ]
 [...]

 Tests passed: 116
 Tests failed:   0

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20240814111005.955359-3-idosch@nvidia.com


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