Bring mlab_export_test uptodate with mlab_export.
Bring mlab_export_test uptodate with mlab_export.

This change brings mlab_export_test uptodate with changes to
mlab_export. This includes removing references to LastTime class in
favor of get_mtime and update_mtime methods as well as the LockFile
class. mlab_export_test defines a static FAKE_TIMESTAMP in favor of
using time.time() or inline constants.

stephen-soltesz/collectd-mlab,m-lab/collectd-mlab,stephen-soltesz/collectd-mlab,m-lab/collectd-mlab