adds diff back on error

[jakecoffman]

This is another attempt to add diffs back to the CLI output when a test fails. See #42 for a previous try.

This time I've added it to the very end of the run, as if you had run diff against the output result and the input. Here's a small example:

INFO Results:
+-----------------------------------------------------------+
|            Changes to Dependabot Pull Requests            |
+---------+-------------------------------------------------+
| created | github.com/fatih/color ( from 1.7.0 to 1.13.0 ) |
+---------+-------------------------------------------------+
time="2023-03-10T16:07:13Z" level=info msg="15/145 calls cached (10%)"
--- ../smoke-tests/tests/smoke-go.yaml
+++ result.txt
@@ -97,7 +97,7 @@
     - type: create_pull_request
       expect:
         data:
-            base-commit-sha: 941c9223edd97d1234567890a404d038a4bc846c4
+            base-commit-sha: 941c9223edd97d233737435a404d038a4bc846c4
             dependencies:
                 - name: github.com/fatih/color
                   previous-requirements:
@@ -227,6 +227,7 @@
                 Bumps [github.com/fatih/color](https://github.com/fatih/color) from 1.7.0 to 1.13.0.
                 - [Release notes](https://github.com/fatih/color/releases)
                 - [Commits](https://github.com/fatih/color/compare/v1.7.0...v1.13.0)
+            grouped-update: false
     - type: mark_as_processed
       expect:
         data:

2023/03/10 10:07:13 update failed expectations

Previous attempts put the diff inline which made it hard to follow what was going on.