Fix test execution on FreeBSD versions other than 9.x

The tests use a plugin based on the name from sys.platform.
Unfortunately that string includes the major version number in
Python 2.7, so the tests would look for builder_freebsd9.py,
builder_freebsd10.py, etc.

The issue doesn't affect Linux as Python returns 'linux2' also
on Linux 3.x -- see http://bugs.python.org/issue12326 for details.

It seems later versions of Python will drop the major version
number, so adopt this convention now for FreeBSD.