How to run tests on a user's database?

[krlmlr]

• Expose default DBItest context from package
• Support updating a context (e.g., database credentials, tweaks)
• Summary of results and failure details
• Documentation

We already have test_all(), we need a convention to retrieve the default context from a database backend. Perhaps a dbiTestContext() method? As a stop gap, looking up an unexported function might be enough.