gdb/testsuite: fix gdb.threads/stepi-random-signal.exp pattern (gdb/26532)

gdb/testsuite: fix gdb.threads/stepi-random-signal.exp pattern (gdb/26532)

Commit 1eb8556f5a8b ("gdb: add infrun_debug_printf macro") changed the
debug output format for `set debug infrun 1`.  The test
gdb.threads/stepi-random-signal.exp uses that debug output, and was
updated, but not correctly.  It results in this failure:

    FAIL: gdb.threads/stepi-random-signal.exp: stepi (no random signal)

Fix it by adjusting the pattern in the test.

gdb/testsuite/ChangeLog:

	PR gdb/26532
	* gdb.threads/stepi-random-signal.exp: Update pattern.

Change-Id: If5fa525e9545e32a286effe6a6184358374bd37c