TST: Fix assert_almost_equal error message

Most tests (in pandas' test suite and in general) are of form
`assert_almost_equal(result, expected)`.  Verbose error message was
treating its first argument as expected, this is now fixed.

Previous message was:

```python
err_msg = lambda a, b: 'expected %.5f but got %.5f' % (a, b)
```

But a, the first argument, is actually `actual`, not expected.

Everywhere else, it makes sense to display `"%s != %s" % (a,b)`