logging in test runner / CI : higher log level but only print failing tests

We should do what in-toto does: have the test runner output much more logging but only for failing tests. This should be especially useful for CI but also for developers.

Assuming we hard-code a good log level (see #1093 for potential alternatives), I think we only need roughly this in aggregate_tests.py:

class TestHandler(logging.StreamHandler):
  def __init__(self):
    super().__init__(self)

  @property
  def stream(self):
    return sys.stderr

  @stream.setter
  def stream(self, value):
    pass

if __name__ == '__main__':
  logging.basicConfig(level=logging.INFO, handlers=[TestHandler()])
  # current testrunner code here...

This is because the testrunner buffering will not work with long lived loggers otherwise: the default handler caches sys.stderr value and testrunner wants to modify it between test runs (and buffering is required to only print the failing test output).

Read aggregate_tests.py and the test-runner buffering behavior described in the issue; compare the proposed handler with the in-toto approach. Confirm that higher-level logging is available for failing tests while passing-test output remains buffered, including in CI.