Add general abstract classes for tests, starting with test classes for

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src/MD_STD.php

@@ -419,6 +419,44 @@ final class MD_STD {
 
     }
 
+    /**
+     * Check if a URL is reachable (200) using curl.
+     *
+     * @param string $url URL to validate.
+     *
+     * @return bool
+     */
+    public static function checkUrlIsReachable(string $url):bool {
+
+        if (filter_var($url, FILTER_VALIDATE_URL) === false) {
+            throw new MDInvalidUrl("URL to check (" . $url . ") does not seem to be a valid URL");
+        }
+
+        $ch = curl_init($url);
+        curl_setopt_array($ch, [
+            CURLOPT_AUTOREFERER    => true,
+            CURLOPT_CONNECTTIMEOUT => 5,
+            CURLOPT_ENCODING       => "",
+            CURLOPT_FOLLOWLOCATION => true,
+            CURLOPT_MAXREDIRS      => 1,
+            CURLOPT_NOBODY         => true,
+            CURLOPT_TIMEOUT        => 5,
+            // It's very important to let other webmasters know who's probing their servers.
+            CURLOPT_USERAGENT => 'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:10.0.2) Gecko/20100101 Firefox/10.0.2',
+            CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_2_0,
+            CURLOPT_TCP_FASTOPEN => true,
+        ]);
+        curl_exec($ch);
+        $code = curl_getinfo($ch, CURLINFO_HTTP_CODE);
+        curl_close($ch);
+        if ($code !== 200) {
+            return false;
+        }
+
+        return true;
+
+    }
+
     /**
      * Sets and returns user language based on a session cookie.
      *