Adjust run-tests.php to dynamic extension loading

PR #6787 changed the behavior of the `--EXTENSIONS--` section, so that
not yet loaded extensions are dynamically loaded if possible.  However,
when no tests are specified for the runner, only tests for already
loaded extensions are run, what defeats the purpose of the improvement
of the `--EXTENSIONS--` behavior.  We cater to that by detecting
loadable extensions, and also run their tests.

run-tests.php

@@ -860,9 +860,22 @@ More .INIs  : " , (function_exists(\'php_ini_scanned_files\') ? str_replace("\n"
     }
     @unlink($info_file);
 
-    // load list of enabled extensions
-    save_text($info_file,
-        '<?php echo str_replace("Zend OPcache", "opcache", implode(",", get_loaded_extensions())); ?>');
+    // load list of enabled and loadable extensions
+    save_text($info_file, <<<'PHP'
+        <?php
+        echo str_replace("Zend OPcache", "opcache", implode(",", get_loaded_extensions()));
+        $ext_dir = ini_get("extension_dir");
+        foreach (scandir($ext_dir) as $file) {
+            if (!preg_match('/^(?:php_)?([_a-zA-Z0-9]+)\.(?:so|dll)$/', $file, $matches)) {
+                continue;
+            }
+            $ext = $matches[1];
+            if (!extension_loaded($ext) && @dl($file)) {
+                echo ",", $ext;
+            }
+        }
+        ?>
+    PHP);
     $exts_to_test = explode(',', `$php $pass_options $info_params $no_file_cache "$info_file"`);
     // check for extensions that need special handling and regenerate
     $info_params_ex = [