fix(lint): resolve E600 test module convention issues

Flatten multi-group test suites to single pairs (string * test_case list)
across bpsec, btree, bundle, bytesrw-eio, cfdp, cgr, claude-skills, and
claudeio. Update test.ml runners to wrap suites in lists. Add missing
test_cache.mli. Fix E600 linter to skip standalone executables and
utility libraries that happen to have a test_ prefix by checking whether
the corresponding .ml defines a suite value.