selftests/bpf: tc_links/tc_opts: Unserialize tests

Tests are serialized because they all use the loopback interface.

Replace the 'serial_test_' prefixes with 'test_ns_' to benefit from the
new test_prog feature which creates a dedicated namespace for each test,
allowing them to run in parallel.

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Link: https://lore.kernel.org/r/20250219-b4-tc_links-v2-3-14504db136b7@bootlin.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>