MLBEDSW-3035: Updated StridedSlice checks

Updated supported operator checks for StridedSlice:
- allow negative indices in begin/end values
- added more checks on shapes

Change-Id: I3ac76bfa6b313f0e2250f0749f152fb0e3aa033c
Signed-off-by: Louis Verhaard <louis.verhaard@arm.com>
diff --git a/ethosu/vela/test/testutil.py b/ethosu/vela/test/testutil.py
index d4ae97b..13b6bf4 100644
--- a/ethosu/vela/test/testutil.py
+++ b/ethosu/vela/test/testutil.py
@@ -56,6 +56,14 @@
     return op
 
 
+def create_op(op_type, inputs, output, attrs=dict()):
+    op = Operation(op_type, output.name + "_op")
+    op.inputs = inputs
+    op.outputs = [output]
+    op.attrs = attrs
+    return op
+
+
 def create_subgraph(op_list):
     # Creates subgraph using the given list of operations
     sg = Subgraph()