Btest

* Add pre-commit configuration. (Benjamin Bannier)

* Reformat Python scripts with yapf. (Benjamin Bannier)

* Reformat shell scripts with shellfmt. (Benjamin Bannier)

* Fix a couple issues tagged by shellcheck. (Benjamin Bannier)

* More precise environment filtering in
local-alternative-show-env.test. (Christian Kreibich, Corelight)

* Fix parallel/interleaved execution between the test wrapup
functions (such as testSucceeded) and the progress monitor.
(Christian Kreibich, Corelight)

* Switch Sphinx build test to use "text" output format. (Christian
Kreibich, Corelight)

* Add GitHub action exercising the test suite. (Benjamin Bannier,
Corelight)

* Make `btest-progress` more robust under concurrency. (Benjamin
Bannier, Corelight)

* Fix test `tests.sphinx.run-sphinx` for sphinx-3.5.4. (Benjamin
Bannier, Corelight)

* Show verbose failures when running test target. (Benjamin Bannier,
Corelight)

* Fix misspelled variable name in `btest-sphinx.py`. (Benjamin
Bannier, Corelight)

* Default to `-j 1` to make CTRL-C work consistently. We now always use
multiprocessing with a single process, with one exception: interactive
baseline updates still run directly. (Christian Kreibich, Corelight)

* Avoid leaking processes due to lacking sync manager shutdown.
(Christian Kreibich, Corelight)

* Speed up the threading test since its built-in delays weren't actually
required for the test. (Christian Kreibich, Corelight)

* Add "binary mode" to btest-diff. In binary mode, invoked with -b/--binary,
btest-diff compares test output and baselines for equality only, never
applies canonifiers, and doesn't apply our btest header when updating
baselines. (Christian Kreibich, Corelight)

* GH-36: Fix --abort-on-failure for expected failures. (Robin Sommer,
Corelight)

* Add support for multiple baseline directories. (Robin Sommer, Corelight)

This works be setting the environment variable BTEST_BASELINE_DIR to a
colon-separated list of directories. They will be searched in order when
looking a baseline file to compare against. Updating a baseline will always
put the new content into the first directory. One can now also generally
set a different baseline directory through BTEST_BASELINE_DIR. See the
README for more.

* Require Python >= 3.5. (Jon Siwek, Corelight)

* Canonify outputs when updating baselines via btest-diff.
(Christian Kreibich, Corelight)

This leverages the same canonicalization btest-diff already
applies during test output comparison against baselines also when
updating those baselines (usually via btest -U/-u). This removes a
bunch of noise from the baselines, including personal home
directories, timestamps, etc.

Since btest-diff doesn't know whether the baseline has undergone
this canonicalization, it continues to canonicalize the baseline
prior to diffing. To track whether it has canonicalized a baseline
when updating, btest-diff now also prepends a header to the
generated baseline that warns users about the fact that it is
auto-generated. The presence of this header doubles as a marker
for canonicalization.

* Clean up btest-diff's shell code. (Christian Kreibich, Corelight)