It's better if the tests aren't interactive, since they're intended for
running on CI as well as to give an idea of whether building was
successful, rather than to give an idea of what has gone wrong.
* Ignore build artifacts in 'tests' directory
* Remove unused variables in makefiles
* Add 'bootstrap-clean' rule to delete build artifacts from 'bootstrap' directory
* Add 'distclean' rule to delete all build artifacts from the source tree
Since they might be... This is especially likely for module hashes, and
if we don't get it right, the Racket runtime might fail to write the
buffer. This makes the code buildable with the Racket back end.
