1. mysqltest.cc - added flush to log file after each executed command in a testcase.
2. mtr shows 20 last lines from test case log file if timeout reached.
3. Optimizing the code by Magnus review.
4. It is partially fix bug#40150
Problem: In the mysqltest language, it was not possible to set the current
connection from a variable, and it was not possible to read the current
connection.
Fix: Allow setting the connection from a variable, like:
connection $variable;
and introduce the mysqltest language variable $CURRENT_CONNECTION, which
holds the name of the current connection.
