Fix completion quote, preposing and target calculation bug

[tompng]

Reline's completion target calculation has a bug

require 'reline'
Reline.completion_proc = proc { |target, preposing, postposing|
  puts; p(preposing:, target:, postposing:); exit
}
Reline.readline 'prompt> '

prompt> foo"bar[TAB]
{:preposing=>"foo", :target=>"bar", :postposing=>""}

Preposing is foo, target is bar, and " disappears. This bug is a blocker of fixing #444

This pull request:

prompt> foo"bar[TAB]
{:preposing=>"foo\"", :target=>"bar\"", :postposing=>""}

Preposing is foo". Target is bar". Quote that is appended to target is the root cause of #444.

One test that began failing is deleted because readline-ext does not act like the behavior expected in that test.

[ima1zumi]

LGTM!