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