Disable unstable test (#10920)

paddle/fluid/inference/tensorrt/convert/CMakeLists.txt

 nv_test(test_op_converter SRCS test_op_converter.cc mul_op.cc conv2d_op.cc DEPS ${FLUID_CORE_MODULES})
-nv_test(test_trt_activation_op SRCS test_activation_op.cc activation_op.cc io_converter.cc
-    DEPS ${FLUID_CORE_MODULES} activation_op tensorrt_engine
-    SERIAL)
+# This test is not stable
+# See https://paddleci.ngrok.io/viewLog.html?tab=buildLog&buildTypeId=Paddle_PrCi2&buildId=36834&_focus=8828 
+#nv_test(test_trt_activation_op SRCS test_activation_op.cc activation_op.cc io_converter.cc
+#    DEPS ${FLUID_CORE_MODULES} activation_op tensorrt_engine
+#    SERIAL)
 nv_test(test_io_converter SRCS test_io_converter.cc io_converter.cc DEPS dynload_cuda dynamic_loader lod_tensor)