diff_test should first run an action that produces a diff output, so it's easy to "accept" the patch

Currently our diff_test (as well as the one in rules_diff) runs `diff` to produce a patch as an ephemeral command during a test action. This means that tooling can't find the resulting patch file and offer to apply it.

Start by locating the diff_test and rules_diff implementations mentioned in the issue, then inspect how their test actions invoke diff and where action outputs are declared. Done means the diff-producing action leaves a patch file that tooling can discover and offer to apply, rather than producing it only as an ephemeral command result.

Written by the indexing model from the issue text.