Add file function test

tests/exercises/echo-function-file/solution/correct.cpp

@@ -0,0 +1,26 @@
+#include <iostream>
+#include <fstream>
+#include <string>
+#include <algorithm>
+#include <cctype>
+
+std::string trim(const std::string& str) {
+    std::string trimmed = str;
+    trimmed.erase(trimmed.begin(), std::find_if(trimmed.begin(), trimmed.end(), [](int ch) {
+        return !std::isspace(ch);
+    }));
+    trimmed.erase(std::find_if(trimmed.rbegin(), trimmed.rend(), [](int ch) {
+        return !std::isspace(ch);
+    }).base(), trimmed.end());
+    return trimmed;
+}
+
+std::string echo_file(const std::string& filename) {
+    std::ifstream file(filename);
+    if (!file) {
+        return "Error: Failed to open file.";
+    }
+
+    std::string content((std::istreambuf_iterator<char>(file)), std::istreambuf_iterator<char>());
+    return trim(content);
+}