Uniquefy gdb.threads/attach-into-signal.exp

Uniquefy gdb.threads/attach-into-signal.exp

Hi,

While examining BuildBot's logs, I noticed:

  <https://sourceware.org/ml/gdb-testers/2015-q3/msg03767.html>

gdb.threads/attach-into-signal.exp has two nested loops and don't use
unique messages.  This commit fixes that.  Pushed under the obvious
rule.

gdb/testsuite/ChangeLog:
2015-07-29  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.threads/attach-into-signal.exp (corefunc): Use
	with_test_prefix on nested loops, uniquefying the test messages.