Set dynamic library path on Windows/OSX before running system tests.

Fixes trac issue [#10816](http://trac.mantidproject.org/mantid/ticket/10816)

It is sufficient to check that the [develop_systemtests_all](http://builds.mantidproject.org/job/develop_systemtests_all/) job has started successfully on Windows & OS X. The job currently has some failing tests but this is not related to this fix. Before this fix the tests were not starting at all on Windows.