Update rawexec and rkt stop/kill tests with the patterns introduced in
7a49e9b68e519050a0c2ef0b67c33503bfbc51be. This implementation should be
more resilient to discrepancy between task stopping and task being marked as exited.
this allows us to drop a cyclical import, but is subobptimal as it
requires BaseDriver tests to move. This falls firmly into the realm of
being a hack. Alternatives welcome.