add simple runnableExamples for atomics (#16116)

lib/pure/concurrency/atomics.nim

@@ -11,6 +11,46 @@
 ##
 ## Unstable API.
 
+runnableExamples:
+  # Atomic
+  var loc: Atomic[int]
+  loc.store(4)
+  assert loc.load == 4
+  loc.store(2)
+  assert loc.load(moRelaxed) == 2
+  loc.store(9)
+  assert loc.load(moAcquire) == 9
+  loc.store(0, moRelease)
+  assert loc.load == 0
+
+  assert loc.exchange(7) == 0
+  assert loc.load == 7
+
+  var expected = 7
+  assert loc.compareExchange(expected, 5, moRelaxed, moRelaxed)
+  assert expected == 7
+  assert loc.load == 5
+
+  assert not loc.compareExchange(expected, 12, moRelaxed, moRelaxed)
+  assert expected == 5
+  assert loc.load == 5
+
+  assert loc.fetchAdd(1) == 5
+  assert loc.fetchAdd(2) == 6
+  assert loc.fetchSub(3) == 8
+
+  loc.atomicInc(1)
+  assert loc.load == 6
+
+  # AtomicFlag
+  var flag: AtomicFlag
+
+  assert not flag.testAndSet
+  assert flag.testAndSet
+  flag.clear(moRelaxed)
+  assert not flag.testAndSet
+
+
 import macros
 
 when defined(cpp) or defined(nimdoc):