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

bin/varnishtest/tests/u00011.vtc

@@ -26,7 +26,7 @@ process p1 -write "vcl.li\t\r"
 
 process p1 -expect-text 0 1 "active      auto/warm"
 
-process p1 -write "vcl.s\t\th\tvcl1\r"
+process p1 -write "vcl.s\t\th\t vcl1\r"
 
 process p1 -expect-text 0 1 "backend s1"