Local (testport) builds were failing like so:
ld: error: undefined symbol: testing::Message::GetString() const
>>> referenced by base.cc
>>> tests/CMakeFiles/mxnet_unit_tests.dir/cpp/misc/base.cc.o:(testing::AssertionResult::AppendMessage(testing::Message const&))
because the testing libraries were not being linked -- because
they're not even searched for when testing is not enabled.
Extend previous patch that put the searching-for-testlibs
behind a check, to apply the same check to building-tests.
1b240c8d4d