xhr-fetch-polyfill: Minor test improvements (#26932)
xhr-fetch-polyfill: Minor test improvements (#26932)

Expect console errors.
sharethrough/amphtml,prateekbh/amphtml,nexxtv/amphtml,voyagegroup/amphtml,lannka/amphtml,honeybadgerdontcare/amphtml,media-net/amphtml,honeybadgerdontcare/amphtml,jmadler/amphtml,media-net/amphtml,alanorozco/amphtml,jridgewell/amphtml,voyagegroup/amphtml,taboola/amphtml,smartadserver/amphtml,alanorozco/amphtml,alanorozco/amphtml,rsimha-amp/amphtml,adup-tech/amphtml,alanorozco/amphtml,jridgewell/amphtml,zhouyx/amphtml,lannka/amphtml,smartadserver/amphtml,nexxtv/amphtml,jmadler/amphtml,alanorozco/amphtml,prateekbh/amphtml,lannka/amphtml,zhouyx/amphtml,cory-work/amphtml,sharethrough/amphtml,mhc-gh/amphtml,zhouyx/amphtml,ampproject/amphtml,danielrozenberg/amphtml,Gregable/amphtml,jmadler/amphtml,lannka/amphtml,danielrozenberg/amphtml,jpettitt/amphtml,jridgewell/amphtml,yandex-pcode/amphtml,jpettitt/amphtml,rsimha-amp/amphtml,yandex-pcode/amphtml,alanorozco/amphtml,media-net/amphtml,jridgewell/amphtml,taboola/amphtml,smartadserver/amphtml,danielrozenberg/amphtml,smartadserver/amphtml,danielrozenberg/amphtml,Gregable/amphtml,jpettitt/amphtml,nexxtv/amphtml,voyagegroup/amphtml,jridgewell/amphtml,mhc-gh/amphtml,zhouyx/amphtml,mhc-gh/amphtml,prateekbh/amphtml,ampproject/amphtml,jpettitt/amphtml,nexxtv/amphtml,alanorozco/amphtml,jmadler/amphtml,adup-tech/amphtml,Gregable/amphtml,taboola/amphtml,jridgewell/amphtml,yandex-pcode/amphtml,cory-work/amphtml,voyagegroup/amphtml,voyagegroup/amphtml,honeybadgerdontcare/amphtml,prateekbh/amphtml,zhouyx/amphtml,Gregable/amphtml,cory-work/amphtml,honeybadgerdontcare/amphtml,cory-work/amphtml,ampproject/amphtml,ampproject/amphtml,adup-tech/amphtml,ampproject/amphtml,cory-work/amphtml,adup-tech/amphtml,media-net/amphtml,mhc-gh/amphtml,zhouyx/amphtml,Gregable/amphtml,rsimha-amp/amphtml,media-net/amphtml,alanorozco/amphtml,sharethrough/amphtml,ampproject/amphtml,Gregable/amphtml,ampproject/amphtml,nexxtv/amphtml,taboola/amphtml,yandex-pcode/amphtml,rsimha-amp/amphtml,voyagegroup/amphtml,prateekbh/amphtml,voyagegroup/amphtml,zhouyx/amphtml,sharethrough/amphtml,adup-tech/amphtml,nexxtv/amphtml,adup-tech/amphtml,yandex-pcode/amphtml,taboola/amphtml,jpettitt/amphtml,mhc-gh/amphtml,cory-work/amphtml,danielrozenberg/amphtml,sharethrough/amphtml,rsimha-amp/amphtml,ampproject/amphtml,rsimha-amp/amphtml,taboola/amphtml,nexxtv/amphtml,media-net/amphtml,jridgewell/amphtml,Gregable/amphtml,sharethrough/amphtml,cory-work/amphtml,danielrozenberg/amphtml,yandex-pcode/amphtml,media-net/amphtml,lannka/amphtml,danielrozenberg/amphtml,Gregable/amphtml,danielrozenberg/amphtml,yandex-pcode/amphtml,media-net/amphtml,jmadler/amphtml,prateekbh/amphtml,mhc-gh/amphtml,honeybadgerdontcare/amphtml,taboola/amphtml,smartadserver/amphtml,jpettitt/amphtml,prateekbh/amphtml,rsimha-amp/amphtml,jpettitt/amphtml,jmadler/amphtml,jpettitt/amphtml,honeybadgerdontcare/amphtml,lannka/amphtml,mhc-gh/amphtml,smartadserver/amphtml,lannka/amphtml,mhc-gh/amphtml,honeybadgerdontcare/amphtml