selftests/bpf: Optionally open a dedicated namespace to run test in it

Some tests are serialized to prevent interference with others.

Open a dedicated network namespace when a test name starts with 'ns_' to
allow more test parallelization. Use the test name as namespace name to
avoid conflict between namespaces.

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Link: https://lore.kernel.org/r/20250219-b4-tc_links-v2-2-14504db136b7@bootlin.com


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