view CONTRIBUTING @ 41682:1ab6f5df263e

test: stabilize test-wireproto-exchangev2.t flaky output

When running the test suite with multiple processes, we often get flaky
outputs, like here: https://ci.octobus.net/job/MercurialPy2/267/console

```
-  received frame(size=0; request=1; stream=2; streamflags=; type=command-response; flags=eos)
   add changeset cd2534766bec
   add changeset e96ae20f4188
   add changeset caa2a465451d
+  received frame(size=0; request=1; stream=2; streamflags=; type=command-response; flags=eos)
```

Instead of simply sorting the clone and pull output, I saved the output in a
separate file and checked the `received frame` messages on one side and then
the rest of the output on the other side. This way we conserve the order of
messages as it seems important.

Differential Revision: https://phab.mercurial-scm.org/D5897

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