liblog: don't sleep in the middle of tests

A lot of liblog tests follow this pattern:
1) Write a log message
2) Sleep ~1 second
3) Use the non_blocking log reader to dump all log messages
4) Test those log messages

This causes running back to back tests to be very slow and still
allows for some amount of flakiness if the system is very loaded.

This change replaces that pattern with the following:
1) Write a log message
2) Set an alarm for 2 seconds as a test timeout
3) Read logs with the blocking reader until finding the expected log
   messages
4) Test those log messages
5) Use the non_blocking reader to dump all log messages
6) Re-test those log messages, to ensure no duplicates, etc, which
   isn't done in step 3).

Despite dumping the logs twice, the tests are orders of magnitude
faster in the good case, and should be less prone to flakes.

Test: liblog-unit-tests
Change-Id: Iedf473316576b8007746fe3560815bde1813787a