[test] enforce bundled pysqlcipher in benchmark tests

The debian package for libsqlcipher does not enable the use of usleep(), and that might cause timeouts when doing concurrent access to sqlcipher databases.

See: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=859685

In benchmark tests we use twisted's asynchronous dbapi for creating thousands of documents, and if usleep() is not enabled this operation is very likelly to fail with a "db is locked" error.

This commit adds a workaround for benchmark tests by reinstalling pysqlcipher with the "--bundled" option, what causes libsqlcipher to be built and bundled in the python package, with usleep() enabled.

Resolves: #8835 (closed)