Add system_env mock & implementation

Author: gaggle

lib/system_env_behaviour.ex

@@ -0,0 +1,7 @@
+defmodule SystemEnvBehaviour do
+  @moduledoc """
+  The abstract behaviour of SystemEnvBehaviour
+  """
+  @callback get_env() :: %{optional(String.t()) => String.t()}
+  @callback get_env(String.t(), String.t() | nil) :: String.t() | nil
+end

lib/system_env_impl.ex

@@ -0,0 +1,17 @@
+defmodule SystemEnvImpl do
+  @moduledoc """
+  An implementation of SystemEnvBehaviour by proxying into System
+  """
+
+  @behaviour SystemEnvBehaviour
+
+  @impl SystemEnvBehaviour
+  def get_env do
+    System.get_env()
+  end
+
+  @impl SystemEnvBehaviour
+  def get_env(varname, default \\ nil) do
+    System.get_env(varname, default)
+  end
+end

test/test_helper.exs

@@ -1 +1,4 @@
+Mox.defmock(SystemEnvBehaviourMock, for: SystemEnvBehaviour)
+Application.put_env(:context, :system_env, SystemEnvBehaviourMock)
+
 ExUnit.start()