add tests for nim-lang#15584 (nim-lang#15619)

* add test for nim-lang#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

tests/stdlib/trlocks.nim

@@ -0,0 +1,14 @@
+discard """
+  action: "compile"
+  # Disallow joining to ensure it can compile in isolation.
+  # See #15584
+  joinable: false
+  cmd: "nim $target --threads:on $options $file"
+"""
+
+# bugfix #15584
+
+import rlocks
+
+var r: RLock
+r.initRLock()

tests/stdlib/tuserlocks.nim

@@ -0,0 +1,13 @@
+discard """
+  cmd: "nim $target --threads:on $options $file"
+"""
+
+import rlocks
+
+var r: RLock
+r.initRLock()
+doAssert r.tryAcquire()
+doAssert r.tryAcquire()
+r.release()
+r.release()
+r.deinitRLock()