switch test framework to pytest

Pytest is a testing framework that is backwards compatible with
"unittest" tests, but provides new styles of tests that are more
ergonomic.

Pytest tests do not require wrapping the test in a class, just a top
level python function will be automatically picked up. Assertions use
the regular python assert built-in and provide greatly enhanced debug
output. These features reduce friction in writing new unit tests, and being
backwards compatible allows preserving the existing gentoolkit unit
tests.

Changing the meson test command and installing the pytest package in CI are the
only changes required to start using it!

.github/workflows/ci.yml

@@ -33,7 +33,7 @@ jobs:
         python -m site
         python -m pip install --upgrade pip
         # setuptools needed for 3.12+ because of https://github.com/mesonbuild/meson/issues/7702.
-        python -m pip install meson ninja setuptools
+        python -m pip install meson ninja setuptools pytest
     - name: Install portage
       run: |
         mkdir portage

meson.build

@@ -39,12 +39,8 @@ endif
 subdir('bin')
 subdir('pym')
 
-test(
-    'python-unittest',
-    py,
-    args : ['-m', 'unittest', 'discover', meson.current_source_dir() / 'pym'],
-    timeout : 0
-)
+pytest = find_program('pytest')
+test('pytest', pytest, args : ['-v', meson.current_source_dir() / 'pym'])
 
 if get_option('code-only')
     subdir_done()