Add variable substitutions for !include (#3510)

tests/unit_tests/test_yaml_util.py

@@ -0,0 +1,13 @@
+from esphome import yaml_util
+from esphome.components import substitutions
+
+
+def test_include_with_vars(fixture_path):
+    yaml_file = fixture_path / "yaml_util" / "includetest.yaml"
+
+    actual = yaml_util.load_yaml(yaml_file)
+    substitutions.do_substitution_pass(actual, None)
+    assert actual["esphome"]["name"] == "original"
+    assert actual["esphome"]["libraries"][0] == "Wire"
+    assert actual["esphome"]["board"] == "nodemcu"
+    assert actual["wifi"]["ssid"] == "my_custom_ssid"