Add a test for poll with null fds.

I don't think we want to *document* this, but since at least
external/curl/lib/select.c is relying on this, we should be careful not
to break it.

Bug: http://b/111850071
Test: ran tests
Change-Id: Ic877250d755a0cb59e77ae38ae5e3949f753dad6
diff --git a/tests/Android.bp b/tests/Android.bp
index ccd1126..fa66f93 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -112,6 +112,7 @@
         "netinet_ip_icmp_test.cpp",
         "netinet_udp_test.cpp",
         "nl_types_test.cpp",
+        "poll_test.cpp",
         "pthread_test.cpp",
         "pty_test.cpp",
         "regex_test.cpp",