Fix the order of some test assertions

Several [exercism/xelixir][xel] assertions seem to be flip-flopped, making
their failure message a bit confusing.

The [`assert/1` macro][as] will format failing values in a helpful manner of
comparing actual and expected values. Given a script with a failing test:

```elixir

defmodule Math do
  def add(x, y), do: 0
end

ExUnit.start

defmodule MathTest do
  use ExUnit.Case, async: true

  test "addition works" do
    assert Math.add(1, 1) == 2
  end
end
```

…running `elixir example.exs` produces a helpful failure message:

```
  1) test addition works (MathTest)
     ** (ExUnit.ExpectationError)
                  expected: 0
       to be equal to (==): 2
     at example.exs:11

Finished in 0.02 seconds (0.02s on load, 0.00s on tests)
1 tests, 1 failures
```

The proper form of writing an assertion seems to be:

```elixir
assert actual_value == expected_value
```

Several [exercism/xelixir][xel] tests of swapped the actual and expected
values. For example, if you look at the [the first test of
point_mutations_test.exs][pntmut], the `actual_value` is on the
right-hand-side of the `==`, which produces a confusing failure message:

```
  1) test no difference between empty strands (DNATest)
     ** (ExUnit.ExpectationError)
                  expected: 0
       to be equal to (==): nil
     at point_mutations_test.exs:14
```

The message is not describing the correct failure of
`DNA.hamming_distance/2`, namely, that it's an empty implementation
that's returning `nil` when the caclulation should return 0. Instead,
this message leads the implementer to believe that the function is
returning 0 when `nil` is what one wants.

Switching up the order of these assertions improves the failure messages
and makes it easier to TDD an exercise solution.

[xel]: https://github.com/exercism/xelixir
[as]: http://elixir-lang.org/docs/stable/ExUnit.Assertions.html#assert/1
[pntmut]: https://github.com/exercism/xelixir/blob/8584070f5d8436816c9d7b1480b687947b383bc3/point-mutations/point_mutations_test.exs#L13-L15
Merge pull request #5 from andrewsardone/assertion-order

Fix the order of some test assertions