Add Test lesson

Closes #3

lessons/testing.md

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+# Testing
+
+Testing is an important part of developing software.  In this lesson we'll look at how to test our Elixir code with ExUnit and some best practices for doing so.
+
+## Table of Contents
+
+- [ExTest](#extest)
+	- [assert](#assert)
+	- [refute](#refute)
+	- [assert_raise](#assert_raise)
+- [Test Setup](#test-setup)
+- [Mocking](#mocking)
+- [Best Practices](#best-practices)
+
+## ExTest
+
+Elixir's builtin test framework in ExUnit, it provides everything you need to thoroughly test your code.  One thing to note before moving on is that all of our tests will be Elixir scripts so we need to use the `.exs` file extension.
+
+Before we can actually we run our test code we need to start ExUnit with `ExUnit.start()`, this is most commonly done in `test/test_helper.exs`.
+
+When we generated our project mix was helpful enough to create a simple test for us, we can find it at `test/concoction_test.exs`:
+
+```elixir
+defmodule ConcoctionTest do
+  use ExUnit.Case
+
+  test "the truth" do
+    assert 1 + 1 == 2
+  end
+end
+```
+
+To run our tests we use `mix test`.  If we do that now we should see an output similar to:
+
+```bash
+Finished in 0.03 seconds (0.02s on load, 0.01s on tests)
+1 tests, 0 failures
+```
+
+### assert
+
+If you've written tests before then you're familiar with `assert`; in some frameworks `should` or `expect` fill the role of `assert`.  
+
+We use the `assert` macro to test that the expression is true.  In the event that it is not, an error will be raise and our tests will fail.  To test a failure let's change our sample and then run `mix test`:
+
+```elixir
+defmodule ConcoctionTest do
+  use ExUnit.Case
+
+  test "the truth" do
+    assert 1 + 1 == 3
+  end
+end
+```
+
+Now we should see a very different kind of output:
+
+```bash
+  1) test the truth (ConcoctionTest)
+     test/concoction_test.exs:4
+     Assertion with == failed
+     code: 1 + 1 == 3
+     lhs:  2
+     rhs:  3
+     stacktrace:
+       test/concoction_test.exs:5
+
+......
+
+Finished in 0.03 seconds (0.02s on load, 0.01s on tests)
+1 tests, 1 failures
+```
+
+ExUnit will tells us exactly where our failed assertions are, what the expected value was, and what the actual was.
+
+### refute
+
+`refute` is to `assert` as `unless` is to `if`.  Use `refute` when you want to ensure an statement is always false.
+
+### assert_raise
+
+Sometimes it may be necessary to assert that an error was been raised, we can do this with `assert_raise`.  We'll see an example of `assert_raise` in the next lesson on Plug.
+
+## Test Setup
+
+In some instances it may be necessary to perform some setup before we run our test or maybe we need some initial values.  To accomplish this we can use the `setup` and `setup_all` macros, `setup` is run before each test and `setup_all` before the suite.  It is expected that they will return a tuple of `{:ok, state}`, the state will be available to our tests.
+
+For the sake of example, we'll change our code to use `setup_all`:
+
+```elixir
+defmodule ConcoctionTest do
+  use ExUnit.Case
+
+  setup_all do
+    {:ok, number: 2}
+  end
+
+  test "the truth", state do
+    assert 1 + 1 == state[:number]
+  end
+end
+```
+
+
+## Mocking
+
+The simple answer to mocking in Elixir: don't.  You may instintually reach for mocks but they are highly discouraged in the Elixir community and for good reason.  If you follow good design principles and best practices the resulting code will be is easy to test as individual components.
+
+Resist the urge.
+
+## Best Practices