Make test runner work with AR test tool
Make test runner work with AR test tool

Since #29572, test runner will be loaded as minitest's plugin.
Therefore, if specify a value in `Minitest.extensions` before the Minitest
initialization process, the extension will not load and the test runner will
not work.
https://github.com/seattlerb/minitest/blob/44eee51ed9716c789c7cea8a90c131cf736b8915/lib/minitest.rb#L86

Also, load processing of adapter is done before minitest option processing,
so set the adapter in the file so that the `-a` option works.

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