test: add unit test to confirm absence of pageToken param in listBuckets request, which causes the 1000+ buckets infinite pagination loop

Author: eliekozah

google/cloud/storage/internal/rest/stub_test.cc

@@ -39,7 +39,6 @@ using ::testing::Contains;
 using ::testing::ElementsAre;
 using ::testing::Eq;
 using ::testing::HasSubstr;
-using ::testing::IsTrue;
 using ::testing::Matcher;
 using ::testing::Pair;
 using ::testing::ResultOf;