fix(test): relax locale-dependent assertion in format_time test (#313)

Remove hardcoded colon check that fails on non-English locales
where os.date('%X') returns formats like '15时30分' instead of
'15:30:00'.

Author: jensenojs

tests/unit/util_spec.lua

@@ -190,7 +190,7 @@ describe('util.format_time', function()
 
       assert.is_not_nil(result)
       assert.is_string(result)
-      assert.is_true(result:find(':', 1, true) ~= nil)
+      assert.is_true(#result > 0)
     end)
 
     it('does not convert regular second timestamps', function()