util: Add SignalInterrupt class and use in shutdown.cpp

This change helps generalize shutdown code so an interrupt can be
provided to libbitcoinkernel callers. This may also be useful to
eventually de-globalize all of the shutdown code.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>
Co-authored-by: TheCharlatan <seb.kung@gmail.com>

src/qt/test/apptests.cpp

@@ -86,7 +86,6 @@ void AppTests::appTests()
 
     // Reset global state to avoid interfering with later tests.
     LogInstance().DisconnectTestLogger();
-    AbortShutdown();
 }
 
 //! Entry point for BitcoinGUI tests.