Silence error in check-all-fish-files

Our docker/docker_run_tests.sh script runs tests in a container with the fish
source tree mounted as read-only.  We have a hack to speed up repeated runs
of the check-all-fish-files test that assumes the source tree is writable.
Paper over this by silencing the error for now.

  There were no remaining checks left to match stderr:1:
    touch: cannot touch '/fish-source/tests/.last-check-all-files': Read-only file system

tests/checks/check-all-fish-files.fish

@@ -15,7 +15,7 @@ end
 
 # Prevent setting timestamp if any errors were encountered
 if test "$fail_count" -eq 0
-    touch $timestamp_file
+    touch $timestamp_file 2>/dev/null
 end
 
 # No output is good output