Fix race condition in testExpiresAsString
Fix race condition in testExpiresAsString

This race condition is common when running the tests with HHVM + JIT,
due to JIT overhead when not warmed up.

umiyosh/SlimDocDemo,geggleto/Slim,prashantquestionpro/Slim,huang53798584/Slim,abfadel/Slim,lmcro/Slim,1nv4d3r5/Slim,mmauri04/Slim,beealone/Slim,umiyosh/SlimDocDemo,andresvidal/Slim,gabriel403/Slim,danielgsims/Slim,shameerc/Slim,julianromera/Slim,zither/Slim,kanellov/Slim,chezzy/Slim,cesarmarinhorj/Slim,greevex/Slim,szulilin/Slim,lcp0578/Slim,luizdeangeli/Slim,jeremykendall/Slim,anuradhaedirisuriya/Slim,taohaolong/Slim,gencer/Slim,nslod/Slim,swantrace/Slim-1,xMolchy/Slim,FraGoTe/Slim,Alex2147483647/Slim,jenalgit/Slim,matheusgomes17/Slim,pedro2555/Slim,ColourboxDevelopment/Slim,kutanari/Slim,tanuck/Slim,joelgarciajr84/Slim,asaokamei/Slim,dkarvounaris/Slim,moufmouf/Slim,silentworks/Slim,lekoaf/Slim,AI0day/slim,tempbottle/Slim,Vinniefern99/Slim,xiuchanghu/Slim,lornajane/Slim,fsciuti/Slim,bact197736/Slim,makelivedotnet/Slim,EhteshamMehmood/Slim