Reinstate variable reading tests

Something about loading 't/samples' before defining these tests confused
library(tap).  It wasn't running any of them.
Restore variable names on output

When generating output for a variable term, be sure to use the
variable's original name.  Otherwise, we get random variable names
assigned by SWI-Prolog.  Users won't be happy if we rewrite their code
and discard all their variable names.