selftests/bpf: Add test for __nullable suffix in tp_btf

selftests/bpf: Add test for __nullable suffix in tp_btf

Add a tracepoint with __nullable suffix in bpf_testmod, and add cases
for it:

$ ./test_progs -t "tp_btf_nullable"
 #406/1   tp_btf_nullable/handle_tp_btf_nullable_bare1:OK
 #406/2   tp_btf_nullable/handle_tp_btf_nullable_bare2:OK
 #406     tp_btf_nullable:OK
 Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Philo Lu <lulie@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240911033719.91468-3-lulie@linux.alibaba.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>