perf test: Don't signal all processes on system when interrupting tests

perf test: Don't signal all processes on system when interrupting tests

This signal handler loops over all tests on ctrl-C, but it's active
while the test list is being constructed. process.pid is 0, then -1,
then finally set to the child pid on fork. If the Ctrl-C is received
during this point a kill(-1, SIGINT) can be sent which affects all
processes.

Make sure the child has forked first before forwarding the signal. This
can be reproduced with ctrl-C immediately after launching perf test
which terminates the ssh connection.

Fixes: 553d5efeb341 ("perf test: Add a signal handler to kill forked child processes")
Signed-off-by: James Clark <james.clark@linaro.org>
Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20241129151948.3199732-1-james.clark@linaro.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>