tests/.gitignore: add entry for generated file

After a "make check" we end up with the following:

$ git status
On branch master
Your branch is up-to-date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)

	tests/test-block-backend

nothing added to commit but untracked files present (use "git add" to track)

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Fixes: commit ad0df3e0 ("block: test blk_aio_flush() with blk->root == NULL")
Cc: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>