Add tests for <endian.h>.

Also, for the stuff that's also in <netinet/in.h> as real functions,
check that they're there too (and as functions rather than macros,
since that was historically not true).

Bug: http://b/28432448
Test: ran tests
Change-Id: I7e4ae926f7e02de3b6dd38d1953e5b3b43d44f74
diff --git a/tests/Android.bp b/tests/Android.bp
index 2ccdbf8..1cca332 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -58,6 +58,7 @@
         "complex_test.cpp",
         "ctype_test.cpp",
         "dirent_test.cpp",
+        "endian_test.cpp",
         "error_test.cpp",
         "eventfd_test.cpp",
         "fcntl_test.cpp",