selftests/bpf: Add test for bpftool access to read-only protected maps

Add selftest cases that validate bpftool's expected behavior when
accessing maps protected from modification via security_bpf_map.

The test includes a BPF program attached to security_bpf_map with two maps:
- A protected map that only allows read-only access
- An unprotected map that allows full access

The test script attaches the BPF program to security_bpf_map and
verifies that for the bpftool map command:
- Read access works on both maps
- Write access fails on the protected map
- Write access succeeds on the unprotected map
- These behaviors remain consistent when the maps are pinned

Signed-off-by: Slava Imameev <[email protected]>
Reviewed-by: Quentin Monnet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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