CMake: allow both SPIRV-Headers and spirv-headers

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/1057
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 176aa91..f979403 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -23,7 +23,7 @@
   if (TARGET gmock_main)
     message(STATUS "Found Google Mock, building tests.")
   else()
-    message(STATUS "Did not find googletest, tests will not be built."
+    message(STATUS "Did not find googletest, tests will not be built. "
       "To enable tests place googletest in '<spirv-dir>/external/googletest'.")
   endif()
 endif()