gdb/testsuite: add back needed -re clause in gdb_breakpoint

Commit 4b9728be ("gdb: use gdb_test_multiple in gdb_breakpoint") caused,
amongst others:

   (gdb) break 1^M
   No line 1 in the current file.^M
   Make breakpoint pending on future shared library load? (y or [n]) n^M
   (gdb) FAIL: gdb.dwarf2/dw2-main-no-line-number.exp: gdb_breakpoint: set breakpoint at 1
   FAIL: gdb.dwarf2/dw2-main-no-line-number.exp: !$breakpoint_at_missing_lineno_set

This is because it removed one empty -re clause (matching just the
prompt) that is necessary after replying "n" to the pending breakpoint
question.  Add this clause back.

Change-Id: Ibfaa059d58bbea660bc29f0547e2f75c323fcbc6
Approved-By: Tom de Vries <tdevries@suse.de>