Fix race condition in test_self_stop

This test was relying on a sleep to synchronise the parent and child
processes. By having the child be the process that stops itself with
SIGSTOP, instead of the parent, we can actually properly wait for that
using waitpid2 and be notified of the stop.

This use of sleep to synchronise processes is potentially flaky and
caused failures under rr's `--chaos` mode.

test/ruby/test_signal.rb

@@ -310,17 +310,25 @@ class TestSignal < Test::Unit::TestCase
   end
 
   def test_self_stop
-    assert_ruby_status([], <<-'end;')
+    omit unless Process.respond_to?(:fork)
-      begin
+    omit unless defined?(Process::WUNTRACED)
-        fork{
+
-          sleep 1
+    # Make a process that stops itself
-          Process.kill(:CONT, Process.ppid)
+    child_pid = fork do
-        }
+      Process.kill(:STOP, Process.pid)
-        Process.kill(:STOP, Process.pid)
+    end
-      rescue NotImplementedError
+
-        # ok
+    # The parent should be notified about the stop
-      end
+    _, status = Process.waitpid2(child_pid, Process::WUNTRACED)
-    end;
+    assert status.stopped?
+
+    # It can be continued
+    Process.kill(:CONT, child_pid)
+
+    # And the child then runs to completion
+    _, status = Process.waitpid2(child_pid)
+    assert status.exited?
+    assert status.success?
   end
 
   def test_sigwait_fd_unused