Integrate whitelist in imperative shell

test/basic_signup_whitelist_test.exs

@@ -6,7 +6,23 @@ defmodule BasicSignupWhitelistTest do
     System.put_env("SIGNUP_ALLOWED_EMAILS", whitelist)
   end
 
+  setup do
+    # Save original env var
+    original_env = System.get_env("SIGNUP_ALLOWED_EMAILS")
+
+    on_exit(fn ->
+      # Restore original env var after each test
+      case original_env do
+        nil -> System.delete_env("SIGNUP_ALLOWED_EMAILS")
+        value -> System.put_env("SIGNUP_ALLOWED_EMAILS", value)
+      end
+    end)
+
+    :ok
+  end
+
   test "When not set, not whitelisted" do
+    System.delete_env("SIGNUP_ALLOWED_EMAILS")
     refute BasicSignupWhitelist.mail_whitelisted("joe@example.com")
   end