Fix guess_funcname helper function

[joshuadavidthomas]

The helper function guess_funcname is used in a failed test report to grab the function name that the error occurred.

It works fine in the simple tests contained in this project’s test suite but fails on a real world project (a Django test suite from one of my other projects).

It’s a pretty simple and naive implementation (loop backwards to find the nearest line starting with def ). It should probably be swapped out with something a bit more robust (ast parsing?) and properly tested to ensure we don’t have to rely on a fallback value of ‘???’.

Raised by @fruch in #16