Use tornado's AsyncHttpClient for ApiClient

tests/test_api_client.py

@@ -0,0 +1,10 @@
+import pytest
+
+from atst.api_client import ApiClient
+
+
+@pytest.mark.gen_test
+def test_api_client(http_client, base_url):
+    client = ApiClient(base_url)
+    response = yield client.get('')
+    assert response.code == 200