test: dfu: script enhancements

Various misc enhancements to dfu_gadget_test.sh:

* After every write (download), perform a write to a different file
  with different data. This ensures that the DFU buffer's content is
  replaced, so that if the read (upload) succeeds, we know that the
  correct data was actually read from the storage device, rather than
  simply being left over in the DFU buffer. This requires two alt
  setting names to be passed to the script, and a dummy data file to
  be generated by dfu_gadget_test_init.sh.

* Fix the assumption that dfu_gadget_test.sh is run from the directory
  that contains it, by cd'ing to that directory before invoking
  ./dfu_gadget_test_init.sh.

* Use $DIR$RCV_DIR consistently, rather than using plain $RCV_DIR in
  some places.

* Add 959, 961 test file sizes, to be consistent with having one
  more than and one less than all the other "round" sizes 64, 128, and
  4096.

* Remove references to $BKP_DIR from dfu_gadget_test_init.sh, since it
  isn't used.

Signed-off-by: Stephen Warren <swarren@nvidia.com>