add test for newFileStream() opening a missing file

tests/stdlib/tstreams2.nim

@@ -0,0 +1,14 @@
+discard """
+  test newFileStream opening a missing file returns nil
+  file: "tstreams2.nim"
+  output: 'fs is: nil'
+"""
+import streams
+var
+  fs = newFileStream("amissingfile.txt")
+  line = ""
+echo "fs is: ",repr(fs)
+if not isNil(fs):
+  while fs.readLine(line):
+    echo line
+  fs.close()