selftests/bpf: Add tests for writing to nf_conn:mark

Add a simple extension to the existing selftest to write to
nf_conn:mark. Also add a failure test for writing to unsupported field.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/f78966b81b9349d2b8ebb4cee2caf15cb6b38ee2.1662568410.git.dxu@dxuuu.xyz


Signed-off-by: Alexei Starovoitov <ast@kernel.org>