Make gdb_test's question non-optional if specified

Make gdb_test's question non-optional if specified

gdb_test supports handling scenarios where GDB asks a question before
finishing handling some command.  The full prototype of gdb_test is:

  # gdb_test COMMAND PATTERN MESSAGE QUESTION RESPONSE

However, QUESTION is a question that GDB _may_ ask, not one that GDB
_must_ ask:

 # QUESTION is a question GDB may ask in response to COMMAND, like
 #   "are you sure?"
 # RESPONSE is the response to send if QUESTION appears.

If GDB doesn't raise the question, the test still passes.

I think that this is a misfeature.  If GDB regresses and stops asking
a question, the testsuite won't notice.  So I think that if a QUESTION
is specified, gdb_test should ensure it comes out of GDB.

Running the testsuite exposed a number of tests that pass
QUESTION/RESPONSE to GDB, but no question comes out.  The previous
commits fixed them all, so this commit changes gdb_test's behavior.

A related issue is that gdb_test doesn't enforce that if you specify
QUESTION, that you also specify RESPONSE.  I.e., you should pass 1, 2,
3, or 5 arguments to gdb_test, but never 4, or more than 5.  Making
gdb_test detect bogus arguments actually regressed some testcases,
also all fixed in previous commits.

Change-Id: I47c39c9034e6a6841129312037a5ca4c5811f0db