Test gdb.base/random-signal.exp with "attach"

Test gdb.base/random-signal.exp with "attach"

This exposes the issued fixed by 2f99e8fc9cb8:

  https://sourceware.org/ml/gdb-patches/2015-12/msg00423.html

to native debugging as well.

gdb/testsuite/ChangeLog:
2016-01-12  Pedro Alves  <palves@redhat.com>

	* gdb.base/random-signal.exp (do_test): New procedure, with body
	of testcase moved in.
	(top level) Call it twice, once with "run" and once with "attach".