follow up nim-lang#17391 add testcase (nim-lang#17404)

* Revert "make system random work in VM"

* fix nim-lang#17380

* attempt to fix bug

* fix

* better

* fix

* a bit

* fix the leaks

* revert

* fix

* better

* follow up nim-lang#17391

* fix

* Update tchannels.nim

* Update tests/stdlib/tchannels.nim

* Update tchannels.nim

tests/stdlib/tchannels.nim

@@ -0,0 +1,33 @@
+discard """
+  timeout:  5.0 # but typically < 1s
+  disabled: "freebsd"
+  matrix: "--gc:arc --threads:on; --gc:arc --threads:on -d:danger"
+"""
+
+when true:
+  # bug #17380: this was either blocking (without -d:danger) or crashing with SIGSEGV (with -d:danger)
+  import std/[channels, isolation]
+  const
+    N1 = 10
+    N2 = 100
+  var
+    sender: array[N1, Thread[void]]
+    receiver: array[5, Thread[void]] 
+
+  var chan = newChannel[seq[string]](N1 * N2) # large enough to not block
+  proc sendHandler() =
+    chan.send(isolate(@["Hello, Nim"]))
+  proc recvHandler() =
+    template fn =
+      let x = chan.recv()
+    fn()
+
+  template benchmark() =
+    for t in mitems(sender):
+      t.createThread(sendHandler)
+    joinThreads(sender)
+    for t in mitems(receiver):
+      t.createThread(recvHandler)
+    joinThreads(receiver)
+  for i in 0..<N2:
+    benchmark()