Re-enable ersatz's test suite
Re-enable ersatz's test suite

`erstaz-0.4.7` has a working test suite again, per
https://github.com/commercialhaskell/stackage/issues/4571#issuecomment-497970551.

Fixes #4571.
asr/stackage,simonmichael/stackage,fimad/stackage,wereHamster/stackage,stackbuilders/stackage,int-index/stackage,michalkonecny/stackage,massysett/stackage,mrkkrp/stackage,Daniel-Diaz/stackage,athanclark/stackage,clinty/stackage,fpco/stackage,iand675/stackage,spencerjanssen/stackage,soenkehahn/stackage,jonschoning/stackage