proxy [ThreadLocal] via thread-parameter; clojure.test/*testing-vars*

- (proxy [ThreadLocal] [] (initialValue [] body)) now builds a real per-thread
  store backed by a Chez thread-parameter, with a lazy initialValue; .get/.set/
  .remove work. Other proxies stay nil. test.check's no-seed PRNG (next-rng) uses
  one, so gen/sample and gen/generate (and everything built on them) now work.
- clojure.test/*testing-vars* (+ *report-counters*) are bound vars now, so a
  defspec run through its :test metadata / default reporter doesn't hit an unbound
  var.

make test green (+1 corpus row), shakesmoke byte-identical. One re-mint (proxy).

stdlib/clojure/test.clj

@@ -22,6 +22,12 @@
 (def once-fixtures (atom []))
 (def each-fixtures (atom []))
 
+;; clojure.test/*testing-vars* — the stack of vars under test. Real clojure.test
+;; binds it around each test var; test.check's default reporter reads it, so a
+;; defspec run through its :test metadata doesn't blow up on an unbound var.
+(def ^:dynamic *testing-vars* (list))
+(def ^:dynamic *report-counters* nil)
+
 (defn reset-report! []
   (reset! counters {:test 0 :pass 0 :fail 0 :error 0 :fails []})
   (reset! ctx-stack [])