feat, tests: add fixtures for prebuilt catalogs

I don't love the name "prebuilt" but I can't think of a better one.

tests/conftest.py

@@ -1,17 +1,56 @@
 from datetime import datetime
 
 import pytest
+from pytest import FixtureRequest
 
 from pystac import Catalog, Collection, Item
 
-from .utils import ARBITRARY_BBOX, ARBITRARY_EXTENT, ARBITRARY_GEOM
+from .utils import ARBITRARY_BBOX, ARBITRARY_EXTENT, ARBITRARY_GEOM, TestCases
 
 
 @pytest.fixture
 def catalog() -> Catalog:
     return Catalog("test-catalog", "A test catalog")
 
 
+@pytest.fixture
+def prebuilt_catalog_1() -> Catalog:
+    path = TestCases.get_path("data-files/catalogs/test-case-1/catalog.json")
+    return Catalog.from_file(path)
+
+
+@pytest.fixture
+def prebuilt_catalog_2() -> Catalog:
+    path = TestCases.get_path("data-files/catalogs/test-case-2/catalog.json")
+    return Catalog.from_file(path)
+
+
+@pytest.fixture
+def prebuilt_catalog_4() -> Catalog:
+    path = TestCases.get_path("data-files/catalogs/test-case-4/catalog.json")
+    return Catalog.from_file(path)
+
+
+@pytest.fixture
+def prebuilt_catalog_5() -> Catalog:
+    path = TestCases.get_path("data-files/catalogs/test-case-5/catalog.json")
+    return Catalog.from_file(path)
+
+
+@pytest.fixture
+def prebuilt_catalog_6() -> Catalog:
+    path = TestCases.get_path("data-files/catalogs/test-case-6/catalog.json")
+    return Catalog.from_file(path)
+
+
+@pytest.fixture(params=["1", "2", "4", "5", "6"])
+def prebuilt_catalog(request: FixtureRequest) -> Catalog:
+    path = TestCases.get_path(
+        f"data-files/catalogs/test-case-{request.param}/catalog.json"
+    )
+    return Catalog.from_file(path)
+
+
 @pytest.fixture
 def collection() -> Catalog:
     return Collection("test-collection", "A test collection", ARBITRARY_EXTENT)