improvements to mtr to properly process datadir and other paths

Need to improve mtr to properly handle datadir and other paths - this task is a split from MDEV-30157:

1) Running .sh scripts (which can be executed before running the test) must be moved to an earlier stage so that they can create directories or perform other useful actions before the boostrap phase of the test;

2) Changes must be added to allow the user to set a custom location for datadir during a test, both as an option on the command line and via a configuration parameter - so that tests can be created that use a custom datadir location, and also check work with this parameter;

3) Must be added more precise filters for options that require a new bootstrap with rebuilding the database before starting the test;