test: restructure into unit / integration / spec layers + shared harness

Reorganize the flat 49-file test/ into three layers (jpm test recurses, so all
are still discovered):
- test/unit/        white-box component tests (reader, evaluator, types,
                    persistent-map, lazy-seq, macro, interop, compiler)
- test/integration/ cross-cutting + regression batteries (conformance, jank,
                    sci-bootstrap/runtime, features, systematic-coverage, api,
                    core, namespaces, ported clojure suites) and
   .../ports/       ported clojure/cljs test batches pending consolidation
- test/spec/        the behavioral contract (built out in following commits)
- test/support/harness.janet  shared defspec table runner (cases compared via
                    Jolt's own =, with a :throws sentinel) + expect= helpers

Files moved with git mv (history preserved) and import paths fixed for depth.
jpm test green. README Test section updated.

Next: build out test/spec/ to cover the public API area-by-area, mining the
integration batteries and filling gaps.

test/integration/bootstrap-test.janet

@@ -0,0 +1,28 @@
+(use ../../src/jolt/evaluator)
+(use ../../src/jolt/types)
+(use ../../src/jolt/reader)
+(use ../../src/jolt/api)
+
+(def ctx (init))
+(def source (slurp "/Users/yogthos/src/sci/src/sci/impl/macros.cljc"))
+(var s source)
+(var count 0)
+
+(while (> (length (string/trim s)) 0)
+  (def [form rest] (parse-next s))
+  (set s rest)
+  (++ count)
+  (if (not (nil? form))
+    (do
+      (printf "eval form %d..." count)
+      (flush)
+      (eval-form ctx @{} form)
+      (printf " OK\n"))))
+
+(printf "\n%d forms processed\n" count)
+(printf "ns: %s\n" (ctx-current-ns ctx))
+
+(let [ns (ctx-find-ns ctx "sci.impl.macros")]
+  (printf "sci.impl.macros bindings:\n")
+  (loop [[name v] :pairs (ns :mappings)]
+    (printf "  %s: macro=%q\n" name (v :macro))))