title: Is it possible to keep the already existent implementations of an interface?
body: <p>I have an interface like this:</p>
  
  <pre><code>interface Database {
      fun insertItems(items: List&lt;ItemData&gt;)
      fun deleteItems(items: List&lt;ItemData&gt;)
      fun runTransaction(alsoDelete: Boolean) {
          insertItems(listOf(ItemData(id = 1), ItemData(id = 2), ItemData(id = 3)))
          if (alsoDelete)
              deleteItems(listOf(ItemData(id = 4), ItemData(id = 5), ItemData(id = 6)))
      }
  }
  </code></pre>
  
  <p>I want to use mockk to create instances of <code>Database</code>, but I only want to mock the abstract methods, while keeping <code>runTransaction</code> which does have an implementation that I'd want to test. The result would be that I could then verify like this:</p>
  
  <pre><code>runTransaction(alsoDelete = true)
  verify { insertItems(listOf(ItemData(id = 1), ItemData(id = 2), ItemData(id = 3))) }   
  </code></pre>
  
  <p>Is it possible to achieve this with mockk?</p>