Add comment about portability to get_testdir_base()
Add comment about portability to get_testdir_base()

dmick/teuthology,zhouyuan/teuthology,SUSE/teuthology,caibo2014/teuthology,dmick/teuthology,ceph/teuthology,tchaikov/teuthology,dmick/teuthology,dreamhost/teuthology,michaelsevilla/teuthology,robbat2/teuthology,SUSE/teuthology,ceph/teuthology,zhouyuan/teuthology,t-miyamae/teuthology,ivotron/teuthology,SUSE/teuthology,dreamhost/teuthology,tchaikov/teuthology,ktdreyer/teuthology,robbat2/teuthology,michaelsevilla/teuthology,t-miyamae/teuthology,ivotron/teuthology,yghannam/teuthology,yghannam/teuthology,ktdreyer/teuthology,caibo2014/teuthology