Btest

* Fix for Python 3. (Robin Sommer, Corelight)

* GH-26: Explicitly use multiprocessing "fork" start-method (Jon Siwek, Corelight)

The default start-method on macOS in Python 3.8+ is "spawn", but that
emits a RuntimeError with current btest structuring.

* GH-11: Fix %DIR not being set correctly in cloned tests. (Jon
Siwek, Corelight)

* Sort XML attributes for more output stability across Python
versions. (Jon Siwek, Corelight)

* Use Sphinx logging API directly for compatibility with Sphinx
2.0+. (Jon Siwek, Corelight)

* Add `--trace-file` option for recording Chrome trace file.
(Benjamin Bannier, Corelight)

* Provide a more apt description for btest. (Christopher M. Hobbs,
Corelight)

* New option -z <RETRIES> to retry any failed tests a few times to
see if they might just be unstable. (Dev Bali, Corelight)