Rework Bcsec::Rack.use_in spec using @let@.   #5457.
Rework Bcsec::Rack.use_in spec using @let@.   #5457.

Given the structure of Bcsec::Rack.use_in's spec, this is mostly a
cosmetic change, but I think it's nicer to have to deal with fewer @
symbols over the test code.

NUBIC/aker,NUBIC/aker