More options for deselecting flaky AppDriver values?

It would be great to have some more options to select or deselect inconsistent values from `AppDriver$expect_values()`.  
What if we could pass a custom function to the input/output/export values? 

Something like this: 

```
AppDriver()$expect_values(
  output = \(x){ x[!grepl("flaky_output1", x)] }
)
```
