Work around change in how MiniTest detects SIGINFO
MiniTest 4.7.3 detects the presence of SIGINFO and stores the answer in a constant. It seems that MiniTest 4.7.4 changes this, and instead relies on an info_signal method being implemented on the runner object. In ActiveSupport::Testing::Isolation, we use ProxyTestResult to stand in for the runner object. This object implements `method_missing`, and as such its #info_signal method has a truthy return value. This results in MiniTest trying to install the SIGINFO handler on platforms where SIGINFO does not exists. To fix, I am simply defining an explicit ProxyTestResult#info_signal method.