feat(models): Added TestModel

TestModel is a test structure
class TestModel:
    name: str                    # Test name
    base_url: str                # App URL
    variables: dict              # Variables for test steps
    auth: AuthModel | None       # How to authenticate
    steps: list[StepModel]       # Test steps

models/itm.py

@@ -0,0 +1,6 @@
+class TestModel(BaseModel):
+    name: str = "Unnamed Test"
+    base_url: str = ""
+    variables: dict[str, str] = {}
+    auth: AuthModel | None = None
+    steps: list[StepModel] = []