selftests/bpf: enable (uncomment) all tests in test_libbpf.sh

libbpf is now able to load successfully test_l4lb_noinline.o and
samples/bpf/tracex3_kern.o.

For the test_l4lb_noinline, uncomment related tests from test_libbpf.c
and remove the associated "TODO".

For tracex3_kern.o, instead of loading a program from samples/bpf/ that
might not have been compiled at this stage, try loading a program from
BPF selftests. Since this test case is about loading a program compiled
without the "-target bpf" flag, change the Makefile to compile one
program accordingly (instead of passing the flag for compiling all
programs).

Regarding test_xdp_noinline.o: in its current shape the program fails to
load because it provides no version section, but the loader needs one.
The test was added to make sure that libbpf could load XDP programs even
if they do not provide a version number in a dedicated section. But
libbpf is already capable of doing that: in our case loading fails
because the loader does not know that this is an XDP program (it does
not need to, since it does not attach the program). So trying to load
test_xdp_noinline.o does not bring much here: just delete this subtest.

For the record, the error message obtained with tracex3_kern.o was
fixed by commit e3d91b0c

 ("tools/libbpf: handle issues with bpf ELF
objects containing .eh_frames")

I have not been abled to reproduce the "libbpf: incorrect bpf_call
opcode" error for test_l4lb_noinline.o, even with the version of libbpf
present at the time when test_libbpf.sh and test_libbpf_open.c were
created.

RFC -> v1:
- Compile test_xdp without the "-target bpf" flag, and try to load it
  instead of ../../samples/bpf/tracex3_kern.o.
- Delete test_xdp_noinline.o subtest.

Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>