selftests/bpf: add gre packets testing to flow_dissector

selftests/bpf: add gre packets testing to flow_dissector

The bpf_flow program is able to handle GRE headers in IP packets. Add a
few test data input simulating those GRE packets, with 2 different
cases:
- parse GRE and the encapsulated packet
- parse GRE only

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-v3-6-45b46494f937@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>