Minor codebase improvements suggested by Sonarcloud

- src/modsecurity.cc
  - Replace the redundant type with "auto".
- src/transaction.cc
  - Avoid this unnecessary copy by using a "const" reference.
- test/common/custom_debug_log.cc
  - Use "=default" instead of the default implementation of this special
    member functions.
    - Removed the unnecessary destructor override instead.
  - Annotate this function with "override" or "final".
    - Removed the unnecessary destructor override instead.
  - Remove this "const" qualifier from the return type in all
    declarations.
- test/common/modsecurity_test_context.h
  - Replace the redundant type with "auto".
- test/regression/regression.cc
  - Use the "nullptr" literal.
  - Replace this declaration by a structured binding declaration.
  - Replace "reinterpret_cast" with a safer operation.

src/transaction.cc

@@ -1529,7 +1529,7 @@ std::string Transaction::toOldAuditLogFormat(int parts,
     }
     if (parts & audit_log::AuditLog::HAuditLogPart) {
         audit_log << "--" << trailer << "-" << "H--" << std::endl;
-        for (auto a : m_rulesMessages) {
+        for (const auto &a : m_rulesMessages) {
             audit_log << a.log(0, m_httpCodeReturned) << std::endl;
         }
         audit_log << std::endl;