Add a test for "link rel='next'" kind of link.
Add a test for "link rel='next'" kind of link.

While this is not strictly valid, we don't put the link markup into
`<head>` to avoid adding more code just for those two new tests.

Robinson10240/vimium,Lw-Cui/vimium,PickRelated/vimium,borgified/vimium,gdh1995/vimium,douglas-larocca/vimium,Aleroniponi/vimium,VPashkov/vimium,willsALMANJ/vimium,wildeyes/vimium,zhangmin510/vimium,mrmr1993/vimium,elegantwww/vimium,Aleroniponi/vimium,justalittlenoob/vimium,PickRelated/vimium,Lw-Cui/vimium,tuchangwei/vimium,willsALMANJ/vimium,zhangmin510/vimium,elderbas/vimium,Aleroniponi/vimium,decaffeinate-examples/vimium,douglas-larocca/vimium,mrmr1993/vimium,poacher2k/vimium,bkudria/vimium,mrmr1993/vimium,PrestanceDesign/vimium,poacher2k/vimium,csmalin/vimium,decaffeinate-examples/vimium,smblott-github/vimium,zhangmin510/vimium,decaffeinate-examples/vimium,cyrixhero/vimium,gdh1995/vimium,bkudria/vimium,csmalin/vimium,borgified/vimium,gdh1995/vimium,hhsue/vimium,Robinson10240/vimium,VPashkov/vimium,bkudria/vimium,philc/vimium,cyrixhero/vimium,wildeyes/vimium,poacher2k/vimium,wildeyes/vimium,philc/vimium,csmalin/vimium,smblott-github/vimium,elderbas/vimium,dimatter/vimium,elegantwww/vimium,elegantwww/vimium,crazypenguincode/vimium,douglas-larocca/vimium,justalittlenoob/vimium,dimatter/vimium,VPashkov/vimium,hhsue/vimium,justalittlenoob/vimium,tuchangwei/vimium,PrestanceDesign/vimium,smblott-github/vimium,Robinson10240/vimium,crazypenguincode/vimium,borgified/vimium,philc/vimium,hhsue/vimium,elderbas/vimium,cyrixhero/vimium,PrestanceDesign/vimium,PickRelated/vimium,tuchangwei/vimium,willsALMANJ/vimium,Lw-Cui/vimium,dimatter/vimium