I've reviewed the patch and made my remarks in the review tool.

These tests don't test the central functionality of Percolator nearly enough. We should test, at least:

1) That the text actually went through the filter (and not directly to the Text
widget).
2) That if a filter modifies the text, the modified text appears in the Text
widget.
3) That a filter can stop the event from continuing through the following
filters.
4) That having more than one filter modify the arguments works.
5) That not having any filters works.
6) That this doesn't only work on a Text widget's "insert" and "delete" events.

There are probably more things we should test. Take a look at what this is used
for in practice in the code for ideas.