selftests/bpf: Add ASSERT_OK_FD macro

selftests/bpf: Add ASSERT_OK_FD macro

Add a new dedicated ASSERT macro ASSERT_OK_FD to test whether a socket
FD is valid or not. It can be used to replace macros ASSERT_GT(fd, 0, ""),
ASSERT_NEQ(fd, -1, "") or statements (fd < 0), (fd != -1).

Suggested-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/ded75be86ac630a3a5099739431854c1ec33f0ea.1720515893.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>