This solution worked for me.

eavSetup->addAttribute( \Magento\Catalog\Model\Product::ENTITY, 'test_demo221', [ 'type' => 'int', 'backend' => '', 'frontend' => '', 'label' => 'Test Attribute', 'input' => '', 'class' => '', 'source' => '', 'global' => \Magento\Catalog\Model\ResourceModel\Eav\Attribute::SCOPE_GLOBAL, 'visible' => true, 'required' => false, 'user_defined' => false, 'default' => 0, 'searchable' => false, 'filterable' => false, 'comparable' => false, 'visible_on_front' => false, 'used_in_product_listing' => true, 'unique' => false, 'apply_to' => '' ] );

$ATTRIBUTE_GROUP = 'General';
$attribute_set_id = '';
$attribute_set_name = 'ENTER ATTRIBUTE SET NAME HERE';
$attributeSetList = $this->getAttributeSetList($attribute_set_name);
foreach ($attributeSetList as $attributeSet) {
     $attribute_set_id = $attributeSet['attribute_set_id'];
}
$entityTypeId = $eavSetup->getEntityTypeId(\Magento\Catalog\Model\Product::ENTITY);
$groupId = $eavSetup->getAttributeGroupId($entityTypeId, $attribute_set_id, $ATTRIBUTE_GROUP);

$eavSetup->addAttributeToGroup(
 $entityTypeId,
 $attribute_set_id,
 $groupId,
 'test_demo221', //attribute_code
 null
 );

This solution worked for me.

eavSetup->addAttribute(
        \Magento\Catalog\Model\Product::ENTITY,
        'test_demo221',
        [
            'type' => 'int',
            'backend' => '',
            'frontend' => '',
            'label' => 'Test Attribute',
            'input' => '',
            'class' => '',
            'source' => '',
            'global' => \Magento\Catalog\Model\ResourceModel\Eav\Attribute::SCOPE_GLOBAL,
            'visible' => true,
            'required' => false,
            'user_defined' => false,
            'default' => 0,
            'searchable' => false,
            'filterable' => false,
            'comparable' => false,
            'visible_on_front' => false,
            'used_in_product_listing' => true,
            'unique' => false,
            'apply_to' => ''
        ]
    );

    $ATTRIBUTE_GROUP = 'General';
    $attribute_set_id = '';
    $attribute_set_name = 'ENTER ATTRIBUTE SET NAME HERE';
    $attributeSetList = $this->getAttributeSetList($attribute_set_name);
    foreach ($attributeSetList as $attributeSet) {
         $attribute_set_id = $attributeSet['attribute_set_id'];
    }
    $entityTypeId = $eavSetup->getEntityTypeId(\Magento\Catalog\Model\Product::ENTITY);
    $groupId = $eavSetup->getAttributeGroupId($entityTypeId, $attribute_set_id, $ATTRIBUTE_GROUP);

    $eavSetup->addAttributeToGroup(
     $entityTypeId,
     $attribute_set_id,
     $groupId,
     'test_demo221', //attribute_code
     null
     );