Support test-unit

[soutaro]

Hi! I want to support test-unit tests. I have added some tests to understand how it is working.

So, this is the outline of my plan:

1. Add vscode:test-unit:list and vscode:test-unit:run rake tasks for test-unit tests, which works exactly like minitest version.
2. Add debug_test-unit.rb file.
3. Rename MinitestTests class to something like RakeBasedTests class, and add options to specify test framework name.

Any feedback is welcome.

[connorshea]

Sorry for not responding to this issue, I would be open to this change.