tests: support testing fd inheritance over exec

Add facility for testing how (many) file descriptors survive an exec.
This allows implementing O_CLOEXEC tests.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

tests/test-helpers.c

@@ -23,6 +23,8 @@
 #include <assert.h>
 #include <errno.h>
 #include <dirent.h>
+#include <stdio.h>
+#include <unistd.h>
 
 #include "test-runner.h"
 
@@ -50,3 +52,13 @@ count_open_fds(void)
 	return count;
 }
 
+void
+exec_fd_leak_check(int nr_expected_fds)
+{
+	const char *exe = "./exec-fd-leak-checker";
+	char number[16] = { 0 };
+
+	snprintf(number, sizeof number - 1, "%d", nr_expected_fds);
+	execl(exe, exe, number, (char *)NULL);
+	assert(0 && "execing fd leak checker failed");
+}