log/full: Fix log level unit test

[kasjer]

Unit test was broken in several ways and pass verdict was not reliable.

First of log_offset used for starting walk was never initialized and received random data.
By accident this data had non zero ts_stamp that resulted in expected start from the beginning (it stack was different it could look for timestamp instead).

log_last_walk function that is called on each enumerated entry used uninitialized lo_index value to report last entry index. The assumption that lo_index has means something in this context was also incorrect as this value never changes in walk functions. It looks like original code expected this value to have index of log entry which it never does.

Code also did not expected that very first entry added to the log will have index 0 (which was never modified correctly). To read entry index log_read function must be called.

So now walk callback function does read index of the log.

After fixing incorrect code few more conditions are tested.

• more logs that should not be stored are generated with different log levels
• logs with log level above threshold are also tested
• additional check if index of logs increases