emergency_handlers: increase max handlers to 256

in the ttcn3_dut application many "virtuaL" UE instances are created and
destroyed when executing the tests.

With the previous limit of 12 handlers the application stopped after
a few tests. With the limit raised to 256 we should be able to run
all white_listed TTCN3 tests for the UE without problems.

This fixes #3886

lib/src/support/emergency_handlers.cc

@@ -26,7 +26,7 @@ struct handler_instance {
 
 // Handlers are added in a thread safe manner without using locks to avoid possible issues if a signal is emitted while
 // modifying the callback array.
-static constexpr unsigned    max_handlers = 12;
+static constexpr unsigned    max_handlers = 256;
 static handler_instance      registered_handlers[max_handlers];
 static std::atomic<unsigned> num_handlers;