Issue #3318770: We don't explicitly validate that core has been updated

package_manager/tests/modules/fixture_manipulator/src/FixtureManipulator.php

@@ -190,7 +190,7 @@ class FixtureManipulator {
       // If we're going to be updating the package data, merge the incoming data
       // into what we already have.
       if ($package) {
-        $install_json_package = NestedArray::mergeDeep($data['packages'][$position], $install_json_package);
+        $install_json_package = $install_json_package + $data['packages'][$position];
       }
 
       // Remove the existing package; the array will be re-keyed by
@@ -283,6 +283,19 @@ class FixtureManipulator {
     return $this;
   }
 
+  /**
+   * Modifies core packages.
+   *
+   * @param string $version
+   *   Target version.
+   */
+  public function setCorePackageVersion(string $version): self {
+    $this->setVersion('drupal/core', $version);
+    $this->setVersion('drupal/core-recommended', $version);
+    $this->setVersion('drupal/core-dev', $version);
+    return $this;
+  }
+
   /**
    * Commits the changes to the directory.
    */