tests: ignore missing file with PYTHONDONTWRITEBYTECODE (issue4239)

tests/test-fncache.t

@@ -227,7 +227,7 b' Aborting transaction prevents fncache ch'
   > cmdtable = {}
   > 
   > EOF
-  $ rm "${extpath}c"
+  $ rm -f "${extpath}c"
   $ touch z
   $ hg ci -qAm z
   transaction abort!