tests/util/grub-shell: Correct netboot and file_filter test failure

tests/util/grub-shell: Correct netboot and file_filter test failure

Correct a test failure in netboot_test and file_filter_test caused by an
issue cleaning up the tmp directory created for netboot. Netboot creates
a subdirectory in the tmp folder that causes the rmdir to fail - so
cleanup the subdirectory first.

Fixes: 1d59f39b5f1b (tests/util/grub-shell: Remove the work directory on successful run and debug is not on)
Signed-off-by: Andrew Hamilton <adhamilt@gmail.com>
Tested-by: Leo Sandoval <lsandova@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>