Add spawn test. Hangs most of the time on linux x64 at least.

tests/system/tsysspawn.nim

@@ -0,0 +1,29 @@
+discard """
+  output: '''4
+8'''
+  cmd: "nimrod $target --threads:on $options $file"
+"""
+
+var
+  x, y = 0
+
+proc p1 =
+  for i in 0 .. 1_000_000:
+    discard
+
+  inc x
+
+proc p2 =
+  for i in 0 .. 1_000_000:
+    discard
+
+  inc y, 2
+
+for i in 0.. 3:
+  spawn(p1())
+  spawn(p2())
+
+sync()
+
+echo x
+echo y