Problem: mysqlbinlog_base64 failed sporadically.

Reason: Missing "flush logs" before running $MYSQL_BINLOG,
which could start dumping the log file before server
has finished writting into it.
Fix:
- implementing --force-if-open option to "mysqlbinlog"
- adding --disable-force-if-open to make $MYSQL_BINLOG
  fail on non-closed log files, to garantee that nobody
  will forget "flush logs" in the future.
- adding "flush logs" into all affected tests.