pkg/uroot/bb: test actual code paths rather than duplicating code in test.
pkg/uroot/bb: test actual code paths rather than duplicating code in test.

GanShun/u-root,rminnich/u-root,rminnich/u-root,hugelgupf/u-root,hugelgupf/u-root,u-root/u-root,u-root/u-root,GanShun/u-root,rminnich/u-root,GanShun/u-root,hugelgupf/u-root,u-root/u-root,hugelgupf/u-root,rminnich/u-root