Add spec on what happens when accept(3) is interrupted by SIGVTALRM
Add spec on what happens when accept(3) is interrupted by SIGVTALRM

* Thread#backtrace uses SIGVTALRM on TruffleRuby to unblock that thread,
  other implementations might do something similar.

nobu/rubyspec,ruby/spec,eregon/rubyspec,eregon/rubyspec,ruby/spec,eregon/rubyspec,nobu/rubyspec,ruby/spec,nobu/rubyspec