mariadb/storage/pbxt/mysql-test/main

There are two ways to make an overlay to test a new storage engine: inclusive
and exclusive.

In the inclusive approach one would create have_pbxt.inc, and the corresponding
have_pbxt.opt. And for every test from the main suite that should run in the
overlay, one would need a wrapper like

=====[create.test]======
source have_pbxt.inc;
source create.test;
drop database pbxt;
========================

The last line is needed, because pbxt creates 'pbxt' database automatically,
and we need to remove it to pass the after-test check.

In the exclusive approach, one would create my.cnf or suite.opt - this affects
all tests, so mtr would run *all* tests of the main suite with the new options.
And one would use disabled.def to selectively disable tests and
--init-file option to create the database pbxt before the before-test check
(then we won't need to drop it at the end of the test).