Fix racy output matching in gdb.asm/asm-source.exp

Fix racy output matching in gdb.asm/asm-source.exp

Testing with:
 $ make check-read1 TESTS="gdb.asm/asm-source.exp"

Exposes a testcase bug that can result in racy fails:

 (gdb) PASS: gdb.asm/asm-source.exp: next over foo3
 return
 Make selected stack frame return now? (y or n) y
 n
 #0  main () at /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.asm/asmsrc1.s:53
 53              gdbasm_exit0
 (gdb) FAIL: gdb.asm/asm-source.exp: return from foo2 (got interactive prompt)
 n

The problem is that the "return now\?.*" regex can match partial
output like this:

 "Make selected stack frame return no"

and then we send the 'y' too early, and then the next time around we
hit gdb_test_multiple's internal "got interactive prompt" regex.

Also, note we match "return no" instead of "return now" because the
regex is missing one quote level.

gdb/testsuite/ChangeLog:
2017-11-09  Pedro Alves  <palves@redhat.com>

	* gdb.asm/asm-source.exp ("kill" test): Match the whole query
	output.  Fix '?' match.