add memfiles test that closing a closed memFile is ignored (and raises no [OSError])

tests/stdlib/tmemfiles1.nim

@@ -0,0 +1,13 @@
+discard """
+test that closing a closed file is ignored (no error raised)
+output: nothing
+"""
+import memfiles, os
+var
+  mm: MemFile
+  fn = "test.mmap"
+# Create a new file
+mm = memfiles.open(fn, mode = fmReadWrite, newFileSize = 20)
+mm.close()
+mm.close()
+if fileExists(fn): removeFile(fn)