test(unit): disable JIT when using mocks (#35913)

This fixes the flaky eval/typval_spec.lua tests.

(cherry picked from commit bfa3e111c2)

test/unit/testutil.lua

@@ -347,6 +347,9 @@ local function alloc_log_new()
         end
       end
     end
+    -- JIT-compiled FFI calls cannot call back into Lua, so disable JIT.
+    -- Ref: https://luajit.org/ext_ffi_semantics.html#callback
+    jit.off()
   end
 
   log.set_mocks = child_call(log.set_mocks)