fix repair to unreliable test (#774)

Commit 612b7ad did not adjust a `compute-size-increments` test in
the intended way, because it used the current thread's parameter
vector instead of the measured thread's parameter vector. This new
attempt adds an extra thread parameter, so it checks the intended
repair without having to be lucky with a "thread.ms" test.

mats/misc.ms

@@ -1127,7 +1127,9 @@
                             (let loop ()
                               (unless (check)
                                 (sleep (make-time 'time-duration 10000 0))
-                                (loop))))])
+                                (loop))))]
+             ;; to make sure a thread parameter's content is not counted:
+             [new-param (make-thread-parameter (make-bytevector (* 3 N)))])
         (mutex-acquire m)
         (let* ([th-code
                 ;; we'll measure the difference between data reachable by this
@@ -1150,7 +1152,13 @@
           ;; Wait for thread to become inactive, blocked on the mutex
           (pause-until (lambda () (= 1 (#%$top-level-value '$active-threads))))
           ;; Get thread's size, which should include bstr
-          (let ([pre-sizes (compute-size-increments (list th-code th))])
+          (let ([pre-sizes (compute-size-increments (list th-code
+                                                          th
+                                                          ;; don't count parameters towards the thread's
+                                                          ;; size; in particular, the `$tt-fat` parameter
+                                                          ;; from a "thread.ms" test can have random symbols,
+                                                          ;; some of which may be bound to large values
+                                                          (#%$tc-field 'parameters (#%$thread-tc th))))])
             (mutex-release m)
             ;; Wait for bytevector to be discarded in the thread
             (pause-until (lambda () (unbox saved)))
@@ -1162,18 +1170,16 @@
             ;; Get thread's size, which shouldn't include bstr
             (let ([post-sizes (compute-size-increments (list th-code
                                                              th
-                                                             ;; don't count parameters towards te thread's
-                                                             ;; size; in particluar, the `$tt-fat` parameter
-                                                             ;; from a "thread.ms" test can have random symbols,
-                                                             ;; some of which may be bound to large values
-                                                             (#%$tc-field 'parameters (#%$tc))))])
+                                                             (#%$tc-field 'parameters (#%$thread-tc th))))])
               (printf "SIZES ~s\n" (list pre-sizes N post-sizes))
               (mutex-release m)
               ;; Wait for thread to exit
               (thread-join th)
               ;; Make sure `compute-size-increments` doesn't crash on a
               ;; terminated thread:
               (compute-size-increments (list th))
+              ;; drop extra bytevector from the current thread:
+              (new-param #f)
               ;; Main result: detected size of `bstr` in the thread
               ;; while it was part of the continuation
               (or (eq? (current-eval) interpret) ; interpreter continuation is not precise enough