[Test] Added test function to test verification of MultiThreshold node

tests/test_verify_custom_ops.py

+import onnx
+from finn.custom_op.layout_custom_ops import CustomOp_Layout
+
+def test_verify_layout_custom_ops():
+    m_node = onnx.helper.make_node(
+        "MultiThreshold",
+        ["v", "thresholds"],
+        ["out"],
+        domain="finn",
+        out_scale=2.0,
+        out_bias=-1.0,
+        out_dtype="",
+    ) 
+
+    inst = CustomOp_Layout[m_node.op_type] 
+    inst.verify_layout(m_node) 
+