[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>.

MANIFEST.in

@@ -2,3 +2,4 @@ include LICENSE README.rst version.txt requirements.txt
 include buildout.cfg develop.cfg
 recursive-include doc conf.py *.rst *.png *.ico *.pdf
 recursive-include beat *.checksum *.index *.json *.data
+recursive-include beat/cmdline/test/prefix *.json *.py