selftests/bpf: re-split main function into dedicated tests

selftests/bpf: re-split main function into dedicated tests

The flow_dissector runs plenty of tests over diffent kind of packets,
grouped into three categories: skb mode, non-skb mode with direct
attach, and non-skb with indirect attach.

Re-split the main function into dedicated tests. Each test now must have
its own setup/teardown, but for the advantage of being able to run them
separately. While at it, make sure that tests attaching the bpf programs
are run in a dedicated ns.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20241120-flow_dissector-v3-4-45b46494f937@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>