Improve coverage slightly

By moving test-only code to a test-only region. Also the skip_list
shared lib can depend on libstdc++ etc.

ConflictSet.cpp

@@ -1825,11 +1825,6 @@ Iterator firstGeq(Node *n, const std::span<const uint8_t> key) {
   return {stepwise.n, stepwise.cmp};
 }
 
-Iterator firstGeq(Node *n, std::string_view key) {
-  return firstGeq(
-      n, std::span<const uint8_t>((const uint8_t *)key.data(), key.size()));
-}
-
 struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
 
   void check(const ReadRange *reads, Result *result, int count) {
@@ -2150,6 +2145,11 @@ void checkParentPointers(Node *node, bool &success) {
   }
 }
 
+Iterator firstGeq(Node *n, std::string_view key) {
+  return firstGeq(
+      n, std::span<const uint8_t>((const uint8_t *)key.data(), key.size()));
+}
+
 [[maybe_unused]] int64_t checkMaxVersion(Node *root, Node *node,
                                          int64_t oldestVersion, bool &success,
                                          ConflictSet::Impl *impl) {