selftests/bpf: migrate flow_dissector namespace exclusivity test

Commit a11c397c43d5 ("bpf/flow_dissector: add mode to enforce global BPF
flow dissector") is currently tested in test_flow_dissector.sh, which is
not part of test_progs. Add the corresponding test to flow_dissector.c,
which is part of test_progs. The new test reproduces the behavior
implemented in its shell script counterpart:
- attach a  flow dissector program to the root net namespace, ensure
  that we can not attach another flow dissector in any non-root net
  namespace
- attach a flow dissector program to a non-root net namespace, ensure
  that we can not attach another flow dissector in root namespace

Since the new test is performing operations in the root net namespace,
make sure to set it as a "serial" test to make sure not to conflict with
any other test.

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-v3-7-45b46494f937@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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