Fix racy output matching in gdb.base/multi-attach.exp, gdb.server/ext-{attach, restar...

Fix racy output matching in gdb.base/multi-attach.exp, gdb.server/ext-{attach, restart, ext-run}.exp

This commit fixes this same problem in several places:

  (gdb) PASS: gdb.multi/multi-attach.exp: backtrace 2
  kill
  Kill the program being debugged? (y or n) y
  (gdb) FAIL: gdb.multi/multi-attach.exp: kill inferior 2 (got interactive prompt)

This is just another case of the gdb_test_multiple's internal "got
interactive prompt" pattern matching because the testcase misses
matching enough.

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

	* gdb.multi/multi-attach.exp ("kill" test): Match the whole query
	output.
	* gdb.server/ext-attach.exp ("kill" test): Likewise.
	* gdb.server/ext-restart.exp ("kill" test): Likewise.
	* gdb.server/ext-run.exp ("kill" test): Likewise.
	* gdb.server/ext-wrapper.exp ("kill" test): Likewise.