--debug crashes should not cause assert-* to be like expect-*

[cgay]

Assume you have a test containing these assertions:

  let frag = read-fragment(source-code);
  assert-instance?(<string-fragment>, frag, description);
  assert-equal(want-value, frag.fragment-value, description);

Normally if the first assertion fails the test should terminate, but if you specify --debug crashes or --debug all the first assertion will not terminate the test and the second assertion will be run, causing a potentially confusing error. (In the above case, "No applicable method for fragment-value(#f)".)

The --debug flag should not change the semantics of assert-*, it should just cause the debugger to be entered when they fail.

I can quickly see from the code that the debug?() function is the problem:

              block ()
                test.test-function();
              exception (err :: <assertion-failure>,
                         test: method (c) ~debug?() end)

but I don't want to squirrel on this right now and the correct fix isn't immediately obvious to me.

(I believe there are other bugs in this area, like I'm not sure whether --debug all actually causes assertion failures (not crashes) to enter the debugger. Verify it.)