MAGETWO-55983: Stabilize Extension Manager functional test

Author: Bohdan Korablov

dev/tests/functional/tests/app/Magento/Setup/Test/Constraint/Extension/AssertSelectSeveralExtensions.php

@@ -27,10 +27,26 @@ public function processAssert(InstallGrid $grid, array $extensions)
         $extensions = $grid->selectSeveralExtensions($extensions);
         \PHPUnit_Framework_Assert::assertEmpty(
             $extensions,
-            'Next extensions are not found on the grid: ' . implode(', ', $extensions)
+            'Next extensions are not found on the grid: ' . $this->getExtensionsNames($extensions)
         );
     }
 
+    /**
+     * Get names of extensions.
+     *
+     * @param Extension[] $extensions
+     * @return string
+     */
+    protected function getExtensionsNames(array $extensions)
+    {
+        $result = [];
+        foreach ($extensions as $extension) {
+            $result[] = $extension->getExtensionName();
+        }
+
+        return implode(', ', $result);
+    }
+
     /**
      * Returns a string representation of successful assertion.
      *