Remodel test suite using Python unittests.
Add more thorough test cases for original "quick" test suite.
Add in automatic Valgrind testing for the "quick" test suite on Linux if
Valgrind was detected at build-time.
Add larger suite of assorted .bz2 test files as git submodule.
Updates to the Compiling documentation.
Increased the minimum Meson version to 0.56 because this commit uses the
`fs` module that was introduced in 0.56.
Added Python 3's `pytest` as a required module for running the tests
when using Meson.
The tests may fail if the BZIP or BZIP2 environment variables are set.
To reproduce the issue, run:
BZIP2="--derp" ctest -V
This commit discards those variables before running the tests.
Resolves: https://gitlab.com/bzip2/bzip2/-/issues/31
Fix courtesy of Michael Orlitzky
