Check if mysqli extension is installed.

Gated the DB benchmark section of the server tests so only performed in the mysqli extension is installed.

src/Form/ServerBenchmarksForm.php

@@ -52,24 +52,32 @@ class ServerBenchmarksForm extends FormBase {
       ];
 
       // Get the DB benchmarks.
-      $dbResults = (new DbBenchmarks())->getResults($iterations);
+      if ($dbResults = (new DbBenchmarks())->getResults($iterations)) {
 
-      // Output DB benchmarks as a table.
-      $header = [$this->t('Benchmarks (DB)'), $this->t('Description'), $this->t('Time (seconds)')];
+        // Output DB benchmarks as a table.
+        $header = [$this->t('Benchmarks (DB)'), $this->t('Description'), $this->t('Time (seconds)')];
 
-      $data = [];
+        $data = [];
 
-      foreach ($dbResults as $key => $benchmark) {
-        $data[] = [
-          $benchmark['title'], $benchmark['description'], $benchmark['result'],
+        foreach ($dbResults as $key => $benchmark) {
+          $data[] = [
+            $benchmark['title'], $benchmark['description'], $benchmark['result'],
+          ];
+        }
+
+        $form[] = [
+          '#theme' => 'table',
+          '#header' => $header,
+          '#rows' => $data,
         ];
-      }
 
-      $form[] = [
-        '#theme' => 'table',
-        '#header' => $header,
-        '#rows' => $data,
-      ];
+      } else {
+
+        $form[] = [
+          '#markup' => '<p>' . $this->t('Database benchmarks are unavailable. Check if the PHP mysqli extension is installed.') . '</p>',
+        ];
+
+      }
 
       // Get the Io benchmarks.
       $ioResults = (new IoBenchmarks())->getResults($iterations);