[tests] Add testing of command line substitutions (#12210)

Co-authored-by: J. Nick Koston <nick+github@koston.org>
Co-authored-by: Jonathan Swoboda <154711427+swoboda1337@users.noreply.github.com>

tests/unit_tests/fixtures/substitutions/00-simple_var.approved.yaml

@@ -8,6 +8,8 @@ substitutions:
   position:
     x: 79
     y: 82
+  a: 15
+  b: 20
 
 esphome:
   name: test
@@ -34,3 +36,5 @@ test_list:
   - '{{{"AA"}}}'
   - '"HELLO"'
   - '{ 79, 82 }'
+  - a: 15 should be 15, overridden from command line
+    b: 20 should stay as 20, not overridden

tests/unit_tests/fixtures/substitutions/00-simple_var.input.yaml

@@ -11,6 +11,13 @@ substitutions:
   position:
     x: 79
     y: 82
+  a: 10
+  b: 20
+
+# The following key is only used by the test framework
+# to simulate command line substitutions
+command_line_substitutions:
+  a: 15
 
 test_list:
   - "$var1"
@@ -35,3 +42,5 @@ test_list:
   - ${ '{{{"AA"}}}' }
   - ${ '"HELLO"' }
   - '{ ${position.x}, ${position.y} }'
+  - a: ${a} should be 15, overridden from command line
+    b: ${b} should stay as 20, not overridden

tests/unit_tests/test_substitutions.py

@@ -138,9 +138,11 @@ def test_substitutions_fixtures(
     # Load using ESPHome's YAML loader
     config = yaml_util.load_yaml(source_path)
 
+    command_line_substitutions = config.pop("command_line_substitutions", None)
+
     config = do_packages_pass(config)
 
-    substitutions.do_substitution_pass(config, None)
+    substitutions.do_substitution_pass(config, command_line_substitutions)
 
     resolve_extend_remove(config)
     verify_database_result = verify_database(config)