bectl: tests: use -R <mount> instead of specifying altroot

-R is currently shorthand for cachefile=none, altroot=<mount>. This is
functionally the same, but perhaps more resilient to future changes that
could be necessary that may be added when -R is specified.

(cherry picked from commit de661c9f8652f6a51a6ca83d404d9170404990f8)