Add basic tests for <net/if.h>.

Bug: http://b/26447711
Change-Id: I9bbcdffc07e99c98e367a63de861ddc7b274d579
diff --git a/tests/net_if_test.cpp b/tests/net_if_test.cpp
new file mode 100644
index 0000000..9f38411
--- /dev/null
+++ b/tests/net_if_test.cpp
@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <net/if.h>
+
+#include <errno.h>
+
+#include <gtest/gtest.h>
+
+TEST(net_if, if_nametoindex_if_indextoname) {
+  unsigned index;
+  index = if_nametoindex("lo");
+  ASSERT_NE(index, 0U);
+
+  char buf[IF_NAMESIZE] = {};
+  char* name = if_indextoname(index, buf);
+  ASSERT_STREQ("lo", name);
+}
+
+TEST(net_if, if_nametoindex_fail) {
+  unsigned index = if_nametoindex("this-interface-does-not-exist");
+  ASSERT_EQ(0U, index);
+}