feat(testrun): build and run test targets, reporting pass or fail

Runs each test binary, captures its exit status, and reports per-test timing
alongside an aggregate line.

lib/testrun.ml

@@ -0,0 +1,12 @@
+open Types
+
+type outcome = { name : string; ok : bool; ms : float; output : string; code : int }
+
+let run_one p (t : target) =
+  let exe = Build.test_of p t in
+  let t0 = Unix.gettimeofday () in
+  let code, out = Exec.capture (Array.of_list (exe :: t.args)) in
+  let ms = (Unix.gettimeofday () -. t0) *. 1000.0 in
+  { name = t.name; ok = code = 0; ms; output = out; code }
+
+let tests p = List.filter (fun (t : target) -> t.kind = Test) p.targets