Bound every test with a wall-clock timeout

A runaway test spins at full CPU instead of failing, stalling the whole
run. A debug hook enforces a per-test deadline; the JIT stays off so the
hook is reachable from compiled code.

tests/harness.lua

@@ -1,14 +1,39 @@
 local M = { passed = 0, failed = 0, failures = {} }
 
+local uv = vim.uv or vim.loop
 local current = '?'
 
+M.timeout_ms = tonumber(vim.env.ARTI_PACK_TEST_TIMEOUT_MS) or 5000
+
+-- LuaJIT never reaches a debug hook from inside a compiled trace, so the
+-- interpreter has to stay in charge for the budget below to be enforceable.
+if _G.jit then
+  _G.jit.off()
+  _G.jit.flush()
+end
+
+local HOOK_INTERVAL = 20000
+
+local function bounded(fn)
+  local deadline = uv.hrtime() + M.timeout_ms * 1e6
+  debug.sethook(function()
+    if uv.hrtime() > deadline then
+      debug.sethook()
+      error(string.format('timed out after %dms', M.timeout_ms), 0)
+    end
+  end, '', HOOK_INTERVAL)
+  local ok, err = pcall(fn)
+  debug.sethook()
+  return ok, err
+end
+
 function M.describe(name, fn)
   current = name
   fn()
 end
 
 function M.it(name, fn)
-  local ok, err = pcall(fn)
+  local ok, err = bounded(fn)
   if ok then
     M.passed = M.passed + 1
   else