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.