gtest: disable icc unsued variable warnings

Allows building with MKLDNN_WERROR=on because this fixes

    warning #2415: variable
    "mkldnn::gtest_SimpleSmall_NCHWdeconvolution_test_float_dummy_"
    of static storage duration was declared but never referenced

Author: Roman Dubtsov

tests/CMakeLists.txt

@@ -51,6 +51,8 @@ elseif(WIN32 AND NOT MINGW)
     if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Intel")
         # 1478, 1786: deprecated functions
         append(CMAKE_TEST_CCXX_NOWARN_FLAGS "/Qdiag-disable:1478 /Qdiag-disable:1786")
+        # 2415: unused variable
+        append(CMAKE_TEST_CCXX_NOWARN_FLAGS "/Qdiag-disable:2415")
     else()
         # c4244: conversion with possible loss of data
         # c4996: unsafe / deprecated functions