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 <[e-mail]>
Reviewed-by: Ian Rogers <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Namhyung Kim <[e-mail]>

tools/perf/tests/builtin-test.c

···
 		for (size_t x = 0; x < num_tests; x++) {
 			struct child_test *child_test = child_tests[x];
 
-			if (!child_test)
+			if (!child_test || child_test->process.pid <= 0)
 				continue;
 
 			pr_debug3("Killing %d pid %d\n",