Fix all DUPLICATE problems in testsuite/gdb.perf/

Currently running "make check-perf" shows:

 ...
 # of duplicate test names       6008
 ...

All those duplicate test names come from gdb.perf/skip-command.exp.
This commit fixes them, using with_test_prefix.

gdb/testsuite/
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	* gdb.perf/skip-command.exp (run_skip_bench): Wrap each for
	iteration in with_test_prefix.

Change-Id: I38501cf70bc6b60306ee7228996ee7bcd858dc1b