selftests: netdevsim: Xfail nexthop test on slow machines

A lot of test cases in the file are related to the idle and unbalanced
timers of resilient nexthop groups and these tests are reported to be
flaky on slow machines running debug kernels.

Rather than marking a lot of individual tests with xfail_on_slow(),
simply mark all the tests. Note that the test is stable on non-debug
machines and that with debug kernels we are mainly interested in the
output of various sanitizers in order to determine pass / fail.

Before:

 # make -C tools/testing/selftests KSFT_MACHINE_SLOW=yes \
 	TARGETS=drivers/net/netdevsim TEST_PROGS=nexthop.sh \
 	TEST_GEN_PROGS="" run_tests
 [...]
 # TEST: Bucket migration after idle timer (with delete)               [FAIL]
 #       Group expected to still be unbalanced
 [...]
 not ok 1 selftests: drivers/net/netdevsim: nexthop.sh # exit=1

After:

 # make -C tools/testing/selftests KSFT_MACHINE_SLOW=yes \
 	TARGETS=drivers/net/netdevsim TEST_PROGS=nexthop.sh \
 	TEST_GEN_PROGS="" run_tests
 [...]
 # TEST: Bucket migration after idle timer (with delete)               [XFAIL]
 #       Group expected to still be unbalanced
 [...]
 ok 1 selftests: drivers/net/netdevsim: nexthop.sh

Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20250729160609.02e0f157@kernel.org/
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250804114320.193203-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/drivers/net/netdevsim/nexthop.sh

@@ -1053,6 +1053,6 @@ trap cleanup EXIT
 
 setup_prepare
 
-tests_run
+xfail_on_slow tests_run
 
 exit $EXIT_STATUS