Introduce test whiteboard

This introduces a simple file-like attribute of every single test,
named, you guessed it right, `whiteboard`.

This can be written multiple times during the test run, or a single
time at the very end of the test, as the test writter wishes.

It will be automatically closed (and flushed) when the test ends.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Add new test whiteboard

Add a new test that exercises the test whiteboard.

When run multiplexed, it writes both simple text and binary random
data, in different sizes and number of writes.

Signed-off-by: Cleber Rosa <crosa@redhat.com>