Problem was the parsing of test suite files for various tags and options.
This was done inefficiently, and include files were re-parsed for every
place they were included. This caused a delay of 20 seconds or so before
the first test started to run.

By parsing more efficiently and re-using first parse for subsequent
inclusion of the same file, time spent parsing is reduced to less than
1 second, and start appears instantaneous.

(With this patch, full ./mtr runs in 3 minutes on my laptop (release
build.)

mysql-test/suite/innodb_plugin/t/innodb_bug52663.test:
  Test is fairly slow, so try to avoid getting stuck with it at the end
  while other workers are idle.