add variable support to CreateStubOverCreateMockArgRector

[TomasVotruba]

New use case to flip to createStub():

final class HandleAssignedVariable extends TestCase
{
    public function testThat()
    {
-        $itemMock = $this->createMock(\stdClass::class);
+        $itemMock = $this->createStub(\stdClass::class);


        $items = [
            $itemMock
        ];
    }
}