MDEV-39082 mysql-test: use vardir as datadir in collect_mysqld_features

MDEV-39082 mysql-test: use vardir as datadir in collect_mysqld_features

collect_mysqld_features probes mariadbd with --datadir=., which can
point to the installed mysql-test tree.

During this probe, mariadbd may run the datadir case-sensitivity check
and try to create a *.lower-test file there. This is a bad fit when the
test suite is run from a read-only installed path (e.g. /usr/share/...).

Use $opt_vardir as the datadir for this probe instead, so it runs in a
writable location and avoids writes into the installed test tree.

This upstream change stemmed from discovering during AppArmor testing that the
mariadb-test-run is writing in the /usr/share/.. path:

    apparmor="DENIED" operation="mknod" ... name="/usr/share/mariadb/mariadb-test/mariadb-aa.lower-test" requested_mask="c"

Origin: https://github.com/MariaDB/server/commit/1ecc1d932a0d89d83c7487da6b34717f10a9b7f5.patch
Forwarded: https://github.com/MariaDB/server/pull/4813

Gbp-Pq: Name MDEV-39082-mysql-test-use-vardir-as-datadir-in-collect_my.patch