sim: cris: replace @srcdir@ test extension with $srcdir/$subdir

sim: cris: replace @srcdir@ test extension with $srcdir/$subdir

The common framework supports $srcdir & $subdir replacements already,
so replace the custom @srcdir@ logic with those.  Since the replace
happens in slurp_options that cris already uses, we don't have any
logic to port over there.  We have to duplicate that into the cris
slurp_rv helper though.