fix(test): Only print relevant lines in functional test stack traces. (#5424) r=vladikoff,philbooth
fix(test): Only print relevant lines in functional test stack traces. (#5424) r=vladikoff,philbooth

By default, intern stack traces print all kinds of unimportant line numbers,
dojo, node_modules, etc. Fortunately, these extra lines can be
filtered by passing `filterErrorStack: true` to the intern config.

Not tied to any issue.
mozilla/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server