[tests] Add graph-testing fixtures

Introduce test fixture to simplify graph testing in isolation.

tests/conftest.py

@@ -0,0 +1,32 @@
+from pathlib import Path
+import tempfile
+
+import pytest
+
+from meshroom.core.graph import Graph
+
+
+@pytest.fixture
+def graphWithIsolatedCache():
+    """
+    Yield a Graph instance using a unique temporary cache directory.
+
+    Can be used for testing graph computation in isolation, without having to save the graph to disk.
+    """
+    with tempfile.TemporaryDirectory() as cacheDir:
+        graph = Graph("")
+        graph.cacheDir = cacheDir
+        yield graph
+
+    
+@pytest.fixture
+def graphSavedOnDisk():
+    """
+    Yield a Graph instance saved in a unique temporary folder.
+
+    Can be used for testing graph IO and computation in isolation.
+    """
+    with tempfile.TemporaryDirectory() as cacheDir:
+        graph = Graph("")
+        graph.save(Path(cacheDir) / "test_graph.mg")
+        yield graph