perf tests: Compensate lower sample freq with longer test loop

perf tests: Compensate lower sample freq with longer test loop

Doesn't work for me:

./perf test -v 19
19: Test software clock events have valid period values    :
--- start ---
mmap size 528384B
mmap size 528384B
All (0) samples have period value of 1!
---- end ----
Test software clock events have valid period values: FAILED!

Compensate the lower freq introduced in 67c1e4a53b17 with a longer loop,

Signed-off-by: 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/r/5281D3B8.2030104@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>