tests: add seccomp_notify unit tests for coverage

tests: add seccomp_notify unit tests for coverage

Add unit tests for seccomp_notify.c to improve code coverage:
- test_cleanup_null: Test cleanup handler with NULL
- test_free_null_context: Test freeing NULL context
- test_load_invalid_path: Test loading plugin with invalid relative path
- test_load_nonexistent_plugin: Test loading non-existent plugin
- test_notify_no_seccomp: Test notify function without seccomp support

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Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>