Fix Unit tests to be compatible with php8

Author: anzin

app/code/Magento/Email/Model/Template.php

@@ -252,7 +252,7 @@ public function getProcessedTemplateSubject(array $variables)
         );
 
         try {
-            $processedResult = $processor->setStoreId($storeId)->filter(__($this->getTemplateSubject()));
+            $processedResult = $processor->setStoreId($storeId)->filter(__($this->getTemplateSubject())->render());
         } catch (\Exception $e) {
             $this->cancelDesignConfig();
             throw new \Magento\Framework\Exception\MailException(__($e->getMessage()), $e);

app/code/Magento/Theme/Model/Indexer/Design/Config.php

@@ -174,7 +174,7 @@ protected function prepareFields()
                 $this->data['fieldsets'][$fieldsetName]['fields'][$fieldName]['origin'] =
                     $this->data['fieldsets'][$fieldsetName]['fields'][$fieldName]['origin']
                         ?: $this->data['fieldsets'][$fieldsetName]['fields'][$fieldName]['name'];
-                if ($fieldsetName != 0) {
+                if ((int) $fieldsetName !== 0) {
                     $this->data['fieldsets'][$fieldsetName]['fields'][$fieldName]['name'] =
                         $this->data['fieldsets'][$fieldsetName]['name'] . '_'
                         . $this->data['fieldsets'][$fieldsetName]['fields'][$fieldName]['name'];

lib/internal/Magento/Framework/Filesystem/Driver/Http.php

@@ -250,7 +250,7 @@ protected function getScheme($scheme = null)
      */
     protected function open($hostname, $port)
     {
-        $result = @fsockopen($hostname, $port, $errorNumber, $errorMessage);
+        $result = @fsockopen($hostname, $port, $errorNumber = null, $errorMessage = null);
         if ($result === false) {
             throw new FileSystemException(
                 new \Magento\Framework\Phrase(

lib/internal/Magento/Framework/Filter/Template.php

@@ -9,6 +9,8 @@
  */
 namespace Magento\Framework\Filter;
 
+use Exception;
+use InvalidArgumentException;
 use Magento\Framework\App\ObjectManager;
 use Magento\Framework\Filter\DirectiveProcessor\DependDirective;
 use Magento\Framework\Filter\DirectiveProcessor\ForDirective;
@@ -168,29 +170,34 @@ public function getTemplateProcessor()
      *
      * @param string $value
      * @return string
-     * @throws \Exception
+     * @throws Exception
      */
     public function filter($value)
     {
+        if (!is_string($value)) {
+            //phpcs:ignore Magento2.Exceptions.DirectThrow
+            throw new InvalidArgumentException(__(
+                'Argument \'value\' must be type of string, %1 given.',
+                gettype($value)
+            )->render());
+        }
+
         foreach ($this->directiveProcessors as $directiveProcessor) {
             if (!$directiveProcessor instanceof DirectiveProcessorInterface) {
-                throw new \InvalidArgumentException(
+                throw new InvalidArgumentException(
                     'Directive processors must implement ' . DirectiveProcessorInterface::class
                 );
             }
 
             if (preg_match_all($directiveProcessor->getRegularExpression(), $value, $constructions, PREG_SET_ORDER)) {
                 foreach ($constructions as $construction) {
                     $replacedValue = $directiveProcessor->process($construction, $this, $this->templateVars);
-
                     $value = str_replace($construction[0], $replacedValue, $value);
                 }
             }
         }
 
-        $value = $this->afterFilter($value);
-
-        return $value;
+        return $this->afterFilter($value);
     }
 
     /**

lib/internal/Magento/Framework/Indexer/Test/Unit/MultiDimensionProviderTest.php

@@ -221,14 +221,14 @@ private function getDimensionProviderMock($dimensions)
             ->disableOriginalClone()
             ->disableArgumentCloning()
             ->disallowMockingUnknownTypes()
-            ->setMethods(['getIterator'])
+            ->onlyMethods(['getIterator'])
             ->getMockForAbstractClass();
 
         $dimensionProviderMock->expects($this->any())
             ->method('getIterator')
             ->willReturnCallback(
                 function () use ($dimensions) {
-                    return \SplFixedArray::fromArray($dimensions);
+                    return new \ArrayIterator($dimensions);
                 }
             );
 
@@ -248,7 +248,7 @@ private function getDimensionMock(string $name, string $value)
             ->disableOriginalClone()
             ->disableArgumentCloning()
             ->disallowMockingUnknownTypes()
-            ->setMethods(['getName', 'getValue'])
+            ->onlyMethods(['getName', 'getValue'])
             ->getMock();
 
         $dimensionMock->expects($this->any())