[setup] Add test prefix content to MANIFEST.in

The prefix content is only partially copied in the package which
breaks the online tests done through beat/beat.web>.
4 jobs for add_missing_prefix_in_manifest in 9 minutes and 7 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #185133
docker-build
build_linux_36

00:02:39

55.0%
passed #185134
docker-build
build_linux_37

00:02:38

55.0%
passed #185131
macosx
build_macosx_36

00:09:04

55.0%
passed #185132
macosx
build_macosx_37

00:08:50

55.0%