gdb/testsuite: use gdb_test_no_output when dumping in gdb.base/dump.exp

I don't know if this is true on all platforms, but from what I can see
on Linux, the dump commands don't output anything.  Use
gdb_test_no_output, which should be a bit more robust than checking for
some specific error patterns.

Change-Id: Idc82298c4752ba7808659dfea2f8324c8a97052d
Approved-By: Tom Tromey <tom@tromey.com>