posix: Implement test for isastream

Add tests for `isastream()`

signed-off-by: Gaetan Perrot <gaetanperrotpro@gmail.com>

tests/posix/common/src/stropts.c

@@ -59,4 +59,13 @@ ZTEST(stropts, test_getpmsg)
 	zassert_equal(errno, ENOSYS, "Expected errno ENOSYS, got %d", errno);
 }
 
+ZTEST(stropts, test_isastream)
+{
+	int fd = -1;
+	int ret = isastream(fd);
+
+	zassert_equal(ret, -1, "Expected return value -1, got %d", ret);
+	zassert_equal(errno, ENOSYS, "Expected errno ENOSYS, got %d", errno);
+}
+
 ZTEST_SUITE(stropts, NULL, NULL, NULL, NULL, NULL);

tests/posix/headers/src/stropts_h.c

@@ -23,6 +23,7 @@ ZTEST(posix_headers, test_stropts_h)
 	zassert_not_null((void *)fattach, "fattach is null");
 	zassert_not_null((void *)getmsg, "getmsg is null");
 	zassert_not_null((void *)getpmsg, "getpmsg is null");
+	zassert_not_null((void *)isastream, "isastream is null");
 
 	zassert_true(sizeof(((struct strbuf *)0)->maxlen) > 0, "maxlen size is 0");
 	zassert_true(sizeof(((struct strbuf *)0)->len) > 0, "len size is 0");