Create integration tests to improve reliability when working with jj

[zschreur]

The way we parse jj's cli output is fairly fragile. We do not have great unit tests and in the case of conflicts, we need to read through the jj source code to try and figure out how it may be formatted.

To reduce the fragility, we should have integration tests which create a jj repo, and then run the commands we parse. The hope would be that this would more quickly notify us if output from a jj command changes.

Commands to parse:

• jj files --no-pager
• jj diff --summary --no-pager
• jj resolve --list

In the case of jj resolve, we should ensure that we cover each conflict scenario i.e. 1, 2, 3 sided conflict; conflicts with deleted files; conflicts with renamed files.