bpf, selftests: Fold test_current_pid_tgid_new_ns into test_progs.

Currently tests for bpf_get_ns_current_pid_tgid() are outside test_progs.
This change folds test cases into test_progs.

Changes from v11:

 - Fixed test failure is not detected.
 - Removed EXIT(3) call as it will stop test_progs execution.

Signed-off-by: Carlos Neira <cneirabustos@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210114141033.GA17348@localhost


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