Fix flaky mixins

This improves `Process.kill` (a way to ensure
that we always cleanup after ourselves)
to be prepared to process existing earlier.

spec/lib/gitlab/cluster/mixins/puma_cluster_spec.rb

@@ -40,7 +40,10 @@ describe Gitlab::Cluster::Mixins::PumaCluster do
 
         yield(process.pid)
       ensure
-        Process.kill(:KILL, process.pid) unless process.eof?
+        begin
+          Process.kill(:KILL, process.pid)
+        rescue Errno::ESRCH
+        end
       end
     end
   end

spec/lib/gitlab/cluster/mixins/unicorn_http_server_spec.rb

@@ -54,7 +54,10 @@ describe Gitlab::Cluster::Mixins::UnicornHttpServer do
 
         yield(process.pid)
       ensure
-        Process.kill(:KILL, process.pid) unless process.eof?
+        begin
+          Process.kill(:KILL, process.pid)
+        rescue Errno::ESRCH
+        end
       end
     end
   end