split node tests into standalone files

tests/node_tests/_shared.py

@@ -0,0 +1,9 @@
+import numpy as np
+from backend.data_types import DataField
+
+
+def make_field(data=None, shape=(64, 64), xreal=1e-6, yreal=1e-6):
+    """Create a DataField, optionally from given data or a random field."""
+    if data is None:
+        data = np.random.default_rng(42).standard_normal(shape)
+    return DataField(data=data, xreal=xreal, yreal=yreal, si_unit_xy="m", si_unit_z="m")