Add a test that attempts a backup while a bulk load is happening.

Currently if a table is being bulk loaded when a backup cursor is opened, the cursor_open call will return EBUSY.

It's not clear what the correct solution is. Checkpoint creates a special empty file in this situation. The options I see:
* Have backup skip any files being bulk loaded (not sure if this has metadata consequences?)
* Continue to have backup cursor_open return EBUSY when a table is being bulk loaded.
* Your good idea goes here.

It's also worth noting that this issue isn't specific to bulk loaded files. It will happen any time a handle is open with the WT_DHANDLE_EXCLUSIVE flag. Other cases where we do that are with rename, compact, etc