TST: Make network tests run twice
Might help with issues like #4242

cc @cpcloud

looks good, @jreback ?
 this is just a try it out..right...still don't really know why these tests occasionally fail?
 yep the latest from #4357 is very weird...so yeah maybe wait to merge...should really try to get to the bottom of this
 its a nice fix ...maybe it will work....up2u
 i like this, but i don't think it will fix #4357 (a separate but related issue) ... would certainly help a lot with _consistent_ errors tho so i'm going to merge.
 thanks @jtratner!
 even tho #4357 is consistent from the OP's end, everyone else is having trouble reproducing it...so fyi by consistent i mean reproducible by at least 2 people ... maybe that's too strict...

that's definitely _not_ saying that there isn't an issue there, there certainly is and it needs fixing

just wanted to clarify my statement above
