Bug#39861:

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

mysql-test/lib/mtr_cases.pm

@@ -688,6 +688,7 @@ sub collect_one_test_case {
   my $tinfo= My::Test->new
     (
      name          => "$suitename.$tname",
+     shortname     => $tname,
      path          => "$testdir/$filename",
 
     );