Fix gdb.server/solib-list.exp native-extended-gdbserver regression

Fix gdb.server/solib-list.exp native-extended-gdbserver regression

Commit 221e1a37 (remote non-stop: Process initially stopped threads
before other commands) caused a test regression when testing with the
native-extended-gdbserver board:

  FAIL: gdb.server/solib-list.exp: non-stop 1: non-stop interior stop (timeout)

This "interior stop" now happens before "target remote" prints the
prompt, so we should no longer explicitly expect it.

gdb/testsuite/ChangeLog:
2015-08-20  Pedro Alves  <palves@redhat.com>

	* gdb.server/solib-list.exp: No longer expect an interior stop in
	non-stop mode.