Tests for interactive selection input

Add InteractiveTestHelper to test interactive sessions that use streams
from textstream.h.  Use this to test the behavior of interactive
selection input.  Errors in the input or the help is not tested (yet),
and some corner cases may be missing, but most of the code is now
covered.  Separate set of tests is needed for SelectionOptionManager
prompts (will be added separately).

Remove obsolete special case handling from the interactive parser
(verified to not change any of the now tested behavior).

Change-Id: I448d470bcc240659a380ffa2d3b492949420c64b