v2:
  - Respect the 3 possible return values when invoking
    compare.compare.

v3:
  - Do not use "len()" to check for containers emptiness (Dylan).
  - Use newly added compare result enumeration.
  - Add extension check to the testtrace backend.

v4:
  - Return directly from within the context manager (Dylan).
  - Use os.path.join instead of just concatenating strings (Dylan).
  - Use textwrap.dedent to enhance readability (Dylan).

Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Part-of: <mesa/piglit!353>