Spider: share MTR test cases among different wrappers

Spider bundled with the community server maintains MTR test cases par wrapper basis (see spider/t, spider/oracle). It would be great if we can share the tests among different wrappers.

The directory structure would be the following:

.

└── mysql-test

    └── spider

        ├── common

        ├── native

        │   └── mariadb

        └── none

The test bodies that should be executed in with different wrappers are in the common directory and they are sourced from test files in mariadb and mysql. A test specific to each wrapper can be just placed in the corresponding directory (none is for tests without remote node specification).

We can execute the same test in different configurations by the above approach too.