Add minimalist load test check for testapp

[leplatrem]

It happened several times that issues are discovered through Kinto load tests running on a full stack, using PostgreSQL.

I suggest we add a small load test in the Cliquet code base, ran on TravisCI, for the provided cliquet.tests.testapp.

This would mean:

• Add a testapp configuration file that uses postgresql
• Import the simulation load test of Kinto for the testapp mushroom resource
• Add a Makefile target like this one
• Run it on Travis like this