Empty files may not have a valid checkpoint but may already exist.
Simplify the test for bulk-loaded files we skipped: don't look for a
valid checkpoint, instead check if the file exists, and if it exists,
there's no work to do.  Ref #684.
Turn on all of the tests in test_backup03, drop the number of rows
and increase the cache size so it runs in less than glacial time.
Ref #684.