Fix ColumnStore to run MTR tests in a build directory

ColumnStore test currently do not work in a build directory.
One cannot do just:

make

cd mysql-test

./mtr --suite=columnstore/basic

There are three problems:

1. Dynamic library location

The dynamic library ha_columnstore.so is generated as

storage/columnstore/columnstore/lib/ha_columnstore.so

.
This location is not standard. MTR expects plugin dynamic libraries to be generated in the top plugin directory, which should be storage/columnstore in case of ColumnStore. We should fix CMake files to generate the shared library as

storage/columnstore/ha_columnstore.so

2. Test location

Tests themselves reside inside the storage/columnstore/columnstore/mtr/ directory. This is also not expected. MTR searches for tests inside /mysql-test/ subdirectories in plugins. For example, ConnectSE tests reside in storage/connect/mysql-test/connect.

We should rename

storage/columnstore/columnstore/mtr

to

storage/columnstore/columnstore/mysql-test

in order to respect MariaDB location convention.

3. Test search paths in mysql-test/lib/mtr_cases.pm in the server repository

This problem should be fixed inside the server repository and is reported as MDEV-25393