Provide a loosely typed buffer layer for simple processing of completed tests.

This avoids all of the buffer management and analysis and should be suitable
for most terminal filters.
Add StreamSummary to generate summary statistics and capture failures.

This is equivalent the to behaviour in the TestResult class, but split into a
dedicated helper (so that decorators and the like don't need to waste cycles
generating them).
Add a dedicated interface for controlling an in-progress TestRun.
Add an adapter to convert ExtendedTestResult to StreamResult.

This permits using code that uses any Python unittest test code
with a StreamResult.