perf tests: Use lower sample_freq in sw clock event period test

perf tests: Use lower sample_freq in sw clock event period test

We were using it at 10 kHz, which doesn't work in machines where somehow
the max freq was auto reduced by the kernel:

[root@ssdandy ~]# perf test 19
19: Test software clock events have valid period values    : FAILED!
[root@ssdandy ~]# perf test -v 19
19: Test software clock events have valid period values    :
--- start ---
Couldn't open evlist: Invalid argument
---- end ----
Test software clock events have valid period values: FAILED!
[root@ssdandy ~]#

[root@ssdandy ~]# cat /proc/sys/kernel/perf_event_max_sample_rate
7000

Reducing it to 500 Hz should be good enough for this test and also
shouldn't affect what it is testing.

But warn the user if it fails, informing the knob and the freq tried.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-548rhj1uo6xbwnxa95kw3hqe@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>