IVGCVSW-5787 Add/Update Execute() implementations in RefActivationWorkload

 * Added multithreaded StridedSliceEndToEndTest

Signed-off-by: Finn Williams <Finn.Williams@arm.com>
Change-Id: I4579db7b5959e0a22256f1bda00238c22e611dec
diff --git a/src/backends/reference/workloads/RefConcatWorkload.hpp b/src/backends/reference/workloads/RefConcatWorkload.hpp
index 0be28bb..f4e1aa8 100644
--- a/src/backends/reference/workloads/RefConcatWorkload.hpp
+++ b/src/backends/reference/workloads/RefConcatWorkload.hpp
@@ -15,7 +15,10 @@
 {
 public:
     using BaseWorkload<ConcatQueueDescriptor>::BaseWorkload;
-    virtual void Execute() const override;
+    void Execute() const override;
+    void ExecuteAsync(WorkingMemDescriptor& workingMemDescriptor)  override;
+private:
+    void Execute(std::vector<ITensorHandle*> inputs, std::vector<ITensorHandle*> outputs) const;
 };
 
 } //namespace armnn