perf test: Add a runs-per-test flag

perf test: Add a runs-per-test flag

To detect flakes it is useful to run tests more than once. Add a
runs-per-test flag that will run each test multiple times. Example
output:

```
$ perf test -r 3 lbr -v
122: perf record LBR tests                                           : Ok
122: perf record LBR tests                                           : Ok
122: perf record LBR tests                                           : Ok
```

Update the documentation for the runs-per-test option.

Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: James Clark <james.clark@linaro.org>
Link: https://lore.kernel.org/r/20250110045736.598281-5-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>