Run test suite with -Werror on CI

From time to time there are reports of DeprecationWarning and other warnings in tests. This is mostly due to CI not running with warnings to cause error. Unless it’s executed by someone with -Wall it’s not caught. I think it’s not much expensive and catches issues with tests in this mode. If the warnings are problematic or false positives we can at least restrict the warnings to DeprecationWarning only. I can’t find old threads about this so I thought to open one. Some sample issues