util.promise: Remove some redundant checks, add tests confirming redundancy This lines don't appear to do anything useful, and all tests pass when they are removed. Discovered via mutation testing. I added extra tests to exercise this code, because I wasn't certain that there were no side-effects caused by removal. Everything appears to be fine, thanks to the "pending" check at the start of promise_settle().

For full information on our dependencies, version requirements, and
where to find them, see https://prosody.im/doc/depends

If you have luarocks available on your platform, install the following:

  - luaexpat
  - luasocket
  - luafilesystem
  - luasec