MDEV-18200 MariaBackup - remove symlink requirement from mariabackup.data_directory

[grooverdan]

• [x] The Jira issue number for this PR is: MDEV-18200

Description

Test if this runs on Windows.

How can this PR be tested?

Basing the PR against the correct MariaDB version

• [ ] This is a new feature and the PR is based against the latest MariaDB development branch.
• [X] This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• [X] I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• [X] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

[vaintroub]

The very purpose of this test is to test the DATA DIRECTORY option. If you don't have symlinks (.isl files) enabled, the DATA DIRECTORY option doesn't work. So, isn't skipping this test correct behaviour? Or am I missing something?

DATA DIRECTORY, with .isl files used to work on Windows without problems. Now it does not, and there is no way to enable it. something somewhere got broken.

[LinuxJedi]

DATA DIRECTORY, with .isl files used to work on Windows without problems. Now it does not, and there is no way to enable it. something somewhere got broken.

Sure, but fudging the test to not test that doesn't seem to be the fix. There should probably be a separate MDEV to fix that.