selftests/bpf: migrate bpf flow dissectors tests to test_progs

selftests/bpf: migrate bpf flow dissectors tests to test_progs

test_flow_dissector.sh loads flow_dissector program and subprograms,
creates and configured relevant tunnels and interfaces, and ensure that
the bpf dissection is actually performed correctly. Similar tests exist
in test_progs (thanks to flow_dissector.c) and run the same programs,
but those are only executed with BPF_PROG_RUN: those tests are then
missing some coverage (eg: coverage for flow keys manipulated when the
configured flower uses a port range, which has a dedicated test in
test_flow_dissector.sh)

Convert test_flow_dissector.sh into test_progs so that the corresponding
tests are also run in CI.

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://lore.kernel.org/r/20241120-flow_dissector-v3-13-45b46494f937@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>