This will allow writing new backends more easily and, additionally,
move the testtrace backend out from the framework so it is only used
for actual testing.

v2:
  - Correct several typos in the apitrace backend.
  - Correct comments in the package file.
  - Added init check to the backends.
  - Deal with exceptions from the backends in compare_replay.
  - Deal with possible exceptions in query_traces_yaml.

v3:
  - Do not default to "[]" for calls in the abstract class but do the
    default initialization in the instance code (Dylan).
  - Do not use "len()" to check for containers emptiness (Dylan).
  - Do not use yoda style comparisons (Dylan).
  - Use "format()" to improve text messages readability (Dylan).
  - Combine exceptions detection in compare_replay (Dylan).
  - Let AttributeError raise all the way up (Dylan).
  - Stop using filter and lambda (Dylan).
  - Use always "-1" in the gfxreconstruct backend when checking for
    the last frame call returns a negative value.

v4:
  - Make DumpBackend.dump() an abstractmethod by moving its code to
    the dump_handler decorator, with the rest of the
    logging (Alexandros).

Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com> [v3]
Part-of: <!353>