IVGCVSW-2005: Pull in backend specific unit tests into Android.mk similarly as backend sources

Change-Id: Icafc644b64c591f4dc0e1560d02b806bf053a2da
diff --git a/src/backends/reference/backend.mk b/src/backends/reference/backend.mk
index 4403ea2..9ecb6d7 100644
--- a/src/backends/reference/backend.mk
+++ b/src/backends/reference/backend.mk
@@ -58,3 +58,12 @@
         workloads/RefSplitterUint8Workload.cpp \
         workloads/ResizeBilinear.cpp \
         workloads/Softmax.cpp
+
+# BACKEND_TEST_SOURCES contains the list of files to be included
+# in the Android unit test build (armnn-tests) and it is picked
+# up by the Android.mk file in the root of ArmNN
+
+BACKEND_TEST_SOURCES := \
+        test/RefCreateWorkloadTests.cpp \
+        test/RefLayerSupportTests.cpp \
+        test/RefLayerTests.cpp