selftests/bpf: ns_current_pid_tgid: Use test_progs's ns_ feature

selftests/bpf: ns_current_pid_tgid: Use test_progs's ns_ feature

Two subtests use the test_in_netns() function to run the test in a
dedicated network namespace. This can now be done directly through the
test_progs framework with a test name starting with 'ns_'.

Replace the use of test_in_netns() by test_ns_* calls.

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Link: https://lore.kernel.org/r/20250219-b4-tc_links-v2-4-14504db136b7@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>