selftests/bpf: Enable signature verification for some lskel tests

The test harness uses the verify_sig_setup.sh to generate the required
key material for program signing.

Generate key material for signing LSKEL some lskel programs and use
xxd to convert the verification certificate into a C header file.

Finally, update the main test runner to load this
certificate into the session keyring via the add_key() syscall before
executing any tests. Use the session keyring in the tests with signed
programs.

Signed-off-by: KP Singh <kpsingh@kernel.org>
Link: https://lore.kernel.org/r/20250921160120.9711-6-kpsingh@kernel.org


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