logd: Add logd_test

* Test statistics output, all four main log id types
  are checked to be present, and output is properly
  formatted with header (ascii new-line terminated
  byte count) and trailer (newline formfeed) frames.
* On dev build test dgram_qlen, circumvent if
  liblog benchmarks executed first though.
* Test if kernel and user space loggers are
  present, and if any content has leaked to both.
* test benchmark and use it to verify worst UID pruning

Change-Id: I890c89f64825956f40108de806ed750b0170ac6c