tests: lib: c_lib: tests for C11 call_once()

Add tests for C11 call_once().

Signed-off-by: Christopher Friedt <[email protected]>

tests/lib/c_lib/thrd/src/once.c

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2023, Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include "thrd.h"
+
+#include <stdint.h>
+#include <threads.h>
+
+#include <zephyr/ztest.h>
+
+static size_t number_of_calls;
+static once_flag flag = ONCE_FLAG_INIT;
+
+static void once_func(void)
+{
+	number_of_calls++;
+}
+
+ZTEST(libc_once, test_call_once)
+{
+	zassert_equal(number_of_calls, 0);
+
+	call_once(&flag, once_func);
+	call_once(&flag, once_func);
+	call_once(&flag, once_func);
+
+	zassert_equal(number_of_calls, 1);
+}
+
+ZTEST_SUITE(libc_once, NULL, NULL, NULL, NULL, NULL);