* add test for #15584
* Rename userlocks.nim to trlocks.nim
* add bug comment
* improve cmd
* reference bugfix, not bug
* add test that runs rlocks
* disallow joining
joining with other test cases (e.g., a test case that invokes `RLock.initRLock()`) may cause this test to compile, when the goal is to test if these lines can be compiled in isolation
