Strip out _flatten.

_flatten used to lead to unhelpful output for e.g. simple hashrefs.

Given two hashrefs to compare where neither contains any references as values,
you'd get unhelpful output like:

```
not ok 1 - Compare straightforward hashrefs
```

That style isn't particularly readable or good at highlighting what's different
betwen the two.

After this change, Data::Dumper will always be used, so the output of the same
comparison would be the much more helpful:

```
```
Update tests to expect new output

Now that we run everything through Data::Dumper, these tests need to expect to
see sane dumpered output, not the old relatively unhelpful style.
Update Changes
Merge pull request #5 from bigpresh/fuck_flatten

Always dump, don't use flatten (RT #95446)