Add a test building front-end assets in CicleCI tests (#3636)
Add a test building front-end assets in CicleCI tests (#3636)

A valuable check for PRs like #3628 touching the tooling.
mixxorz/wagtail,FlipperPA/wagtail,rsalmaso/wagtail,rsalmaso/wagtail,wagtail/wagtail,mixxorz/wagtail,gasman/wagtail,nealtodd/wagtail,FlipperPA/wagtail,nimasmi/wagtail,mixxorz/wagtail,rsalmaso/wagtail,mikedingjan/wagtail,rsalmaso/wagtail,timorieber/wagtail,kaedroho/wagtail,takeflight/wagtail,torchbox/wagtail,gasman/wagtail,jnns/wagtail,wagtail/wagtail,iansprice/wagtail,timorieber/wagtail,zerolab/wagtail,zerolab/wagtail,zerolab/wagtail,torchbox/wagtail,kaedroho/wagtail,wagtail/wagtail,torchbox/wagtail,nimasmi/wagtail,kaedroho/wagtail,mikedingjan/wagtail,jnns/wagtail,mixxorz/wagtail,mikedingjan/wagtail,nimasmi/wagtail,thenewguy/wagtail,iansprice/wagtail,nimasmi/wagtail,rsalmaso/wagtail,nealtodd/wagtail,zerolab/wagtail,FlipperPA/wagtail,kaedroho/wagtail,nealtodd/wagtail,jnns/wagtail,thenewguy/wagtail,thenewguy/wagtail,gasman/wagtail,wagtail/wagtail,takeflight/wagtail,iansprice/wagtail,takeflight/wagtail,thenewguy/wagtail,gasman/wagtail,nealtodd/wagtail,timorieber/wagtail,jnns/wagtail,torchbox/wagtail,wagtail/wagtail,timorieber/wagtail,takeflight/wagtail,zerolab/wagtail,kaedroho/wagtail,mikedingjan/wagtail,thenewguy/wagtail,FlipperPA/wagtail,iansprice/wagtail,mixxorz/wagtail,gasman/wagtail