Dropping or renaming tables returns EBUSY in incremental backup test

In the incremental backup test (in test/csuite/incr_backup) , there are two features that are "disabled".  See the variable do_rename and do_drop.  If either of these is set to true, the test consistently fails when WT_SESSION->rename or WT_SESSION->drop is called, as they return EBUSY.  We need to investigate why this occurs - is it an error in the test (perhaps a stray cursor is left open), or an error in the backup implementation.