dnn(test): skip unstable GatherMultiOutput OCL_FP16 test

modules/dnn/test/test_onnx_importer.cpp

@@ -718,6 +718,9 @@ TEST_P(Test_ONNX_layers, Conv1d_variable_weight_bias)
 
 TEST_P(Test_ONNX_layers, GatherMultiOutput)
 {
+    if (cvtest::skipUnstableTests && backend == DNN_BACKEND_OPENCV && target == DNN_TARGET_OPENCL_FP16)
+        throw SkipTestException("Skip unstable test: https://github.com/opencv/opencv/issues/18937");
+
 #if defined(INF_ENGINE_RELEASE)
     if (target == DNN_TARGET_MYRIAD)
         applyTestTag(CV_TEST_TAG_DNN_SKIP_IE_MYRIAD, CV_TEST_TAG_DNN_SKIP_IE);