selftests/bpf: ns_current_pid_tgid: Rename the test function

selftests/bpf: ns_current_pid_tgid: Rename the test function

Next patch will add a new feature to test_prog to run tests in a
dedicated namespace if the test name starts with 'ns_'. Here the test
name already starts with 'ns_' and creates some namespaces which would
conflict with the new feature.

Rename the test to avoid this conflict.

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Link: https://lore.kernel.org/r/20250219-b4-tc_links-v2-1-14504db136b7@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>