Refactor all book.*.test targets into a common target.

Instead of having duplicate, mostly identical targets to test the
various supported interpreters, just move the code into a common target
and have each book.*.test target set ${SCHEME} to the appropriate value
and call ${MAKE} recursively.
Remove extra < char at the beginning of the file.
Add bench.chap7d target.

Allows comparison of the bytecode interpreter from chap7 against the
fast interpretations in chap6.