Use the knots backtrace printer for tests

tests.scm

@@ -1,6 +1,7 @@
 (define-module (tests)
   #:use-module (ice-9 exceptions)
   #:use-module (fibers)
+  #:use-module (knots)
   #:export (run-fibers-for-tests
             assert-no-heap-growth))
 
@@ -15,9 +16,10 @@
                 (simple-format #t "running ~A\n" thunk)
                 (with-exception-handler
                     (lambda (exn)
-                      (backtrace)
+                      (print-backtrace-and-exception/knots exn)
                       (raise-exception exn))
-                  thunk)
+                  (lambda ()
+                    (start-stack #t (thunk))))
                 #t)
               #:unwind? #t))
           #:hz 0