FindGTest: Improve test to catch link error

Add a reference to one of Google Test's command-line flags to the
FindGTest test. This will ensure that we are using the correct compile
definitions on Windows, as the test will otherwise fail to link. (IOW,
this tests the changes made by the previous commit.)

Tests/FindGTest/Test/main.cxx

@@ -2,5 +2,7 @@
 
 TEST(FindCMake, LinksAndRuns)
 {
+  using namespace testing;
+  EXPECT_FALSE(GTEST_FLAG(list_tests));
   ASSERT_TRUE(true);
 }