selftests/bpf: factorize conn and syncookies tests in a single runner

selftests/bpf: factorize conn and syncookies tests in a single runner

btf_skc_cls_ingress currently describe two tests, both running a simple
tcp server and then initializing a connection to it. The sole difference
between the tests is about the tcp_syncookie configuration, and some
checks around this feature being enabled/disabled.

Share the common code between those two tests by moving the code into a
single runner, parameterized by a "gen_cookies" argument. Split the
performed checks accordingly.

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20241020-syncookie-v2-1-2db240225fed@bootlin.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>