UI test output should be `patch` compatible

[nikomatsakis]

@eddyb made the good point that it'd be nice if the diff produced by UI tests were something one could easily feed into patch. Also, the role of - as "expected" and + as "actual" is correct, but the ordering is backwards from what diff uses. It'd be nice to change the call to diff so that instead of doing diff_lines(actual, expected) it did diff_lines(expected, actual) (and then making "left" print - etc).

(Another question, can we add unit tests here somehow?)

cc @cengizio

[cengiz-io]

This can be closed since #42144 has landed.