Make slog test setup more robust

The slog tests fail when attempting to create pools using file vdevs
that already exist from previous test runs. Remove these files in the
setup for the test.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9194