Fail the tests if we detect any AssertionErrors

Fail the tests if we detect any AssertionErrors

This exposed various problems in the recent contextmanagement work,
e.g. the assertions in the __del__ methods had been failing.  Of
course this means that from now on, asserts should only be used to
detect fatal problems.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
[rlb@defaultvalue.org: adjust commit message]
Tested-by: Rob Browning <rlb@defaultvalue.org>