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/unit/eval-test.janet

@@ -0,0 +1,14 @@
+(use ../../src/jolt/api)
+(defn ct-eval [ctx s] (normalize-pvecs (eval-string ctx s)))
+(print "Eval Tests")
+
+(print "1: eval literal...")
+(let [ctx (init)]
+  (assert (= 42 (ct-eval ctx "(eval 42)")) "eval literal")
+  (assert (= 3 (ct-eval ctx "(eval '(+ 1 2))")) "eval quoted form")
+  (assert (= 3 (ct-eval ctx "(eval (eval '(+ 1 2)))")) "eval nested")
+  (ct-eval ctx "(eval '(def ex 99))")
+  (assert (= 99 (ct-eval ctx "ex")) "eval defines var"))
+(print "  ok")
+
+(print "\nAll Eval tests passed!")