tests: pass /dev/null as config for mongod

On certain system, such as Darwin, the default mongod binary is a
wrapper adding a default configuration file. Let's specify that there's
no configuration file to read, otherwise a log file is used instead of
stdout.

Change-Id: I56c72cd61399405fd57d6e3f2b1f53ca6a395835
Merge "tests: pass /dev/null as config for mongod"