selftests/bpf: expose all subtests from flow_dissector

selftests/bpf: expose all subtests from flow_dissector

The flow_dissector test integrated in test_progs actually runs a wide
matrix of tests over different packets types and bpf programs modes, but
exposes only 3 main tests, preventing tests users from running specific
subtests with a specific input only.

Expose all subtests executed by flow_dissector by using
test__start_subtest().

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-v3-5-45b46494f937@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>