perf test hwmon_pmu: Fix event file location

perf test hwmon_pmu: Fix event file location

The temp directory is made and a known fake hwmon PMU created within
it. Prior to this fix the events were being incorrectly written to the
temp directory rather than the fake PMU directory. This didn't impact
the test as the directory fd matched the wrong location, but it
doesn't mirror what a hwmon PMU would actually look like.

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Link: https://lore.kernel.org/r/20241206042306.1055913-2-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>