Add slow tag

Author: eregon

spec/tags/core/signal/trap_tags.txt

@@ -7,3 +7,4 @@ slow:Signal.trap allows to register a handler for all known signals, except rese
 slow(not slow, only fails on JDK10, where SIGUSR1 is no longer reserved):Signal.trap raises ArgumentError for reserved signal: SIGUSR1
 slow:Signal.trap returns 'DEFAULT' for the initial SIGINT handler
 slow:Signal.trap accepts 'SYSTEM_DEFAULT' and uses the OS handler for SIGPIPE
+slow:Signal.trap allows to register a handler for all known signals, except reserved signals for which it raises ArgumentError