Trivial tests for <inttypes.h>/<stdlib.h> *abs and *div functions.

Because I want something to copy & paste into the NDK support library test
that's slightly better than taking the address of the function...

Bug: https://github.com/android-ndk/ndk/issues/502
Test: ran tests
Change-Id: If43089d16691d6a4dcf5d972450b14ed85bbca81
diff --git a/tests/stdlib_test.cpp b/tests/stdlib_test.cpp
index 7d2dc20..27495d8 100644
--- a/tests/stdlib_test.cpp
+++ b/tests/stdlib_test.cpp
@@ -650,3 +650,18 @@
 TEST(stdlib, strtoumax_smoke) {
   CheckStrToInt(strtoumax);
 }
+
+TEST(stdlib, abs) {
+  ASSERT_EQ(INT_MAX, abs(-INT_MAX));
+  ASSERT_EQ(INT_MAX, abs(INT_MAX));
+}
+
+TEST(stdlib, labs) {
+  ASSERT_EQ(LONG_MAX, labs(-LONG_MAX));
+  ASSERT_EQ(LONG_MAX, labs(LONG_MAX));
+}
+
+TEST(stdlib, llabs) {
+  ASSERT_EQ(LLONG_MAX, llabs(-LLONG_MAX));
+  ASSERT_EQ(LLONG_MAX, llabs(LLONG_MAX));
+}