liblog: cleanup TODOs in tests

1) log_msg.msg() will never be nullptr, unless logd sends an invalid
   response, so it's the right idea to ASSERT() that this is true
   instead of just checking and continuing.
2) Even though liblog.too_big_payload is tautological, there's no
   reason not to test it, in case the assumptions that make it
   tautological change.
3) We're not too worried about the return value of logging functions
   or that liblog prevents them from being written (anyone can write
   their own values to logd after all).

Test: liblog-unit-tests
Change-Id: I144cc7cf45c164ea5f04e0786ff0e298fd626f07