Properly handle test cases terminated by signals in test-bundled-gems

Process::Status#exitstatus turn into nil when child process is signeled.
When exit_code was unchanged, test-bundled-gems.rb returned 0 and make was unable to detect the failure. Fix this.

tool/test-bundled-gems.rb

@@ -133,7 +133,7 @@ File.foreach("#{gem_dir}/bundled_gems") do |line|
       puts colorize.decorate(mesg, "skip")
     else
       failed << gem
-      exit_code = $?.exitstatus if $?.exitstatus
+      exit_code = 1
     end
   end
 end