Error log gets truncated when mysqld is restarted by MTR

mysql-test/include/check-warnings.test:
  Since server doesn't have log-error, get it from env. var. set by MTR
mysql-test/lib/My/ConfigFactory.pm:
  "Hide" log-error in a comment
mysql-test/mysql-test-run.pl:
  "Hide" log-error in my.cnf by comment
  add --console to arguments on Windows
  Move .err file to var/log