perf test: Reset shadow counts before loading

Otherwise load counting is an average. Without this change
duration_time in test_memory_bandwidth will alter its value if an
earlier test contains duration_time.

This patch fixes an issue that's introduced in the proposed patch:
https://lore.kernel.org/lkml/20211124015226.3317994-1-irogers@google.com/


in perf test "Parse and process metrics".

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Clarke <pc@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20211128085810.4027314-1-irogers@google.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>