Add helper to wrap result of std::filesystem::path::u8string() (#2535)

In C++17 the return type of std::filesystem::path::u8string() is
std::string, but in C++20 the return type changed to std::u8string.

Add a helper to copy a std::u8string to a std::string, to be used when a
std::string is required. This fixes the build for C++20.

test_common/harness/stringHelpers.h

@@ -39,4 +39,20 @@ inline std::string str_sprintf(const std::string &str, Args... args)
     return std::string(buffer.get(), buffer.get() + s - 1);
 }
 
+// Returns the argument, converted to std::string.
+// The return type of std::filesystem::path::u8string() was
+// std::string in C++17, but became std::u8string in C++20.
+// Use this method to wrap the result when a std::string
+// is desired.
+//
+// Use a template with a specialization for std::string,
+// so the generic template applies when std::u8string exists
+// and is used.
+template <typename STRING_TYPE>
+inline std::string to_string(const STRING_TYPE &str)
+{
+    return std::string(str.begin(), str.end());
+}
+inline std::string to_string(const std::string &str) { return str; }
+
 #endif // STRING_HELPERS_H