work around older libedit which fails to append rl_completion_append_character
work around older libedit which fails to append rl_completion_append_character

I failed to find any issue in varnish and failed to find a
workaround for the code, so change the test to add an additional
space after the tab-completion.

If anyone has a better solution, please go head. Otherwise I feel
this is not worth any more time.

The test change, though not being ideal, should not cause any
harm at least.

Closes #2833

gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache