Is it ok to do this when testing with phpunit?

Is it ok to create a single object instance and storing it in a private or static property?

I mean, most of the examples out there will create one instance per test. In the other hand, this seems like a waste of resources to me.

Here is an example:

use PHPUnit\Framework\TestCase;

class MyClassTest extends TestCase
{
    private $obj;

    public function testFoo()
    {
        $this->assertEquals($this->obj->foo(), 'something');
    }


    public function testBar()
    {
        $this->assertEquals($this->obj->bar(), 'something else');
    }

    public function __construct()
    {
        $this->obj = new MyClass();
    }
}

Is this a bad thing or could it cause any unexpected behaviour?