Add ctermid.

Change-Id: I7c7c815c2725df222932db923632c8b6419741ab
diff --git a/tests/stdio_test.cpp b/tests/stdio_test.cpp
index 7e82612..4db1f72 100644
--- a/tests/stdio_test.cpp
+++ b/tests/stdio_test.cpp
@@ -1297,3 +1297,11 @@
 
   fclose(fp);
 }
+
+TEST(STDIO_TEST, ctermid) {
+  ASSERT_STREQ("/dev/tty", ctermid(nullptr));
+
+  char buf[L_ctermid] = {};
+  ASSERT_EQ(buf, ctermid(buf));
+  ASSERT_STREQ("/dev/tty", buf);
+}