perf test: In forked mode add check that fds aren't leaked

perf test: In forked mode add check that fds aren't leaked

When a test is forked no file descriptors should be open, however,
parent ones may have been inherited - in particular those of the pipes
of other forked child test processes. Add a loop to clean-up/close
those file descriptors prior to running the test. At the end of the
test assert that no additional file descriptors are present as this
would indicate a file descriptor leak.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250624190326.2038704-6-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>