Clean up test framework to no longer use sandbox

- Now we use namespace-anchor->empty-namespace in a new Pyret module,
  then use namespace-require to include copies of all the relevant
  modules

- A bunch of module tests didn't have #lang pyret at the top; this worked
  because of the previous "parse and inline" strategy of module inclusion

- compile.rkt now compiles exprs that are s-prog forms with the appropriate
  requires and provides.  This could add a bit of tension later if we copy
  s-prog expressions around and nest them, because require and provide
  won't be allowed in those contexts