This required moving some tests to "racket-test", and putting them
in modules that use `rackunit`. Also, some uses of rackunit were
removed and switched to more primitive systems. Some improvements
here, particularly in "foreign-test.rktl", would still be good,
but everything passes at the moment.
The test tried to detect separate evaluations by using the
result of `(current-inexact-milliseconds)`, but a clock's
resolution might not be high enough. Use `(gensym)`, instead.
This supports running the core tests on systems which can't run
or don't want to install all the dependencies of the "racket-test"
package.
The "racket-test-core" pkg stil depends on "sandbox-lib" because
the sandbox tests live with the core tests. Hopefully I'll be able
to fix that eventually.
