added a test

tests/typerel/tstr_as_openarray.nim

@@ -0,0 +1,22 @@
+discard """
+  output: '''success'''
+"""
+var s = "HI"
+
+proc x (zz: openarray[char]) =
+  discard
+
+x s
+
+proc z [T] (zz: openarray[T]) =
+  discard
+
+z s
+z([s,s,s])
+
+proc y [T] (arg: var openarray[T]) =
+  arg[0] = 'X'
+y s
+doAssert s == "XI"
+
+echo "success"