Add self-hosting SX test spec: 81 tests bootstrap to Python + JS

The test framework is written in SX and tests SX — the language proves
its own correctness. test.sx defines assertion helpers (assert-equal,
assert-true, assert-type, etc.) and 15 test suites covering literals,
arithmetic, comparison, strings, lists, dicts, predicates, special forms,
lambdas, higher-order forms, components, macros, threading, truthiness,
and edge cases.

Two bootstrap compilers emit native tests from the same spec:
- bootstrap_test.py → pytest (81/81 pass)
- bootstrap_test_js.py → Node.js TAP using sx-browser.js (81/81 pass)

Also adds missing primitives to spec and Python evaluator: boolean?,
string-length, substring, string-contains?, upcase, downcase, reverse,
flatten, has-key?. Fixes number? to exclude booleans, append to
concatenate lists.

Includes testing docs page in SX app at /specs/testing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sxc/pages/docs.sx

@@ -341,6 +341,8 @@
               :filename (get item "filename") :href (str "/specs/" (get item "slug"))
               :source (read-spec-file (get item "filename"))))
         extension-spec-items))
+    "testing" (~spec-testing-content
+      :spec-source (read-spec-file "test.sx"))
     :else (let ((spec (find-spec slug)))
       (if spec
         (~spec-detail-content