QA: Enable all tests by default

This patch enables all tests by default, unless when they're
explicitely disabled in the config file. This will make sure
newly added tests are run even when an old configuration file
is used.

A comment is also added qa-sample.json.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>