Force gtest to expose ::testing::Combine

Work around faulty logic in googletest, where ::testing::Combine
is accidentally disabled for VS 2017.
See https://github.com/google/googletest/issues/1352
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index f979403..fb8c4b5 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -48,6 +48,10 @@
       # We don't care much about that in test code.
       # Important to do since we have warnings-as-errors.
       target_compile_options(${target} PRIVATE /wd4503)
+      # Googletest accidentally turns off support for ::testing::Combine
+      # in VS 2017.  See https://github.com/google/googletest/issues/1352
+      # Forcibly turn it on again.
+      target_compile_options(${target} PRIVATE /DGTEST_HAS_COMBINE=1)
     endif()
     target_include_directories(${target} PRIVATE
       ${SPIRV_HEADER_INCLUDE_DIR}