feat: add multiple new stages (i1 to i5) for inheritance testing and introduce a new fixture for comprehensive test coverage

Author: ryan-gang

internal/test_helpers/course_definition.yml

@@ -5897,3 +5897,8 @@ stages:
   - slug: "dg2"
   - slug: "ou5"
   - slug: "eb9"
+  - slug: "i1"
+  - slug: "i2"
+  - slug: "i3"
+  - slug: "i4"
+  - slug: "i5"