statements missed from general log

A refinement of the test in the previous patch to avoid
using sleep as a means to ensure that timestamps are
added to the log entries.

mysql-test/t/log_tables_debug.test:
  New test file. A debug feature is used to ensure that
  log entries are prefixed with a timestamp.
sql/log.cc:
  A debug feature is implemented to ensure that
  log entries are prefixed with a timestamp.