Fix some migrate tests; drupal_generate_test_ua() can generate warnings, which...

Fix some migrate tests; drupal_generate_test_ua() can generate warnings, which trigger different paths in PHPUnit 9 vs 10.

core/includes/bootstrap.inc

@@ -235,6 +235,9 @@ function drupal_valid_test_ua($new_prefix = NULL) {
 
 /**
  * Generates a user agent string with a HMAC and timestamp for tests.
+ *
+ * @return string|null
+ *   The user agent, or NULL on failure.
  */
 function drupal_generate_test_ua($prefix) {
   static $key, $last_prefix;
@@ -259,7 +262,9 @@ function drupal_generate_test_ua($prefix) {
       // Generate and save a new hash salt for a test run.
       // Consumed by drupal_valid_test_ua() before settings.php is loaded.
       $private_key = Crypt::randomBytesBase64(55);
-      file_put_contents($key_file, $private_key);
+      if (!@file_put_contents($key_file, $private_key)) {
+        return NULL;
+      }
     }
     // The file properties add more entropy not easily accessible to others.
     $key = $private_key . filectime(__FILE__) . fileinode(__FILE__);

core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php

@@ -23,8 +23,8 @@ public function __invoke() {
     // database prefix were stored statically in a file or database variable.
     return function ($handler) {
       return function (RequestInterface $request, array $options) use ($handler) {
-        if ($test_prefix = drupal_valid_test_ua()) {
-          $request = $request->withHeader('User-Agent', drupal_generate_test_ua($test_prefix));
+        if ($user_agent = drupal_generate_test_ua(drupal_valid_test_ua())) {
+          $request = $request->withHeader('User-Agent', $user_agent);
         }
         return $handler($request, $options)
           ->then(function (ResponseInterface $response) {