define and document a policy when to add a new test executable

[bashbaug]

When adding tests for "unified SVM" Ewan asked whether the new tests should be added in their own suite and in a new test executable:

#2174 (review)

We also have tests being added for the "buffer device address" extension, currently in their own test suite:

#2192

Plus, we have tests for existing extensions, sometimes in their own test suite, and sometimes not.

We should define and document a policy when an extension should be added as a new test suite in a new test executable and when tests for an extension should be incorporated into an existing test suite.