meson: Set PYTHONDONTWRITEBYTECODE=1 in test environment

Let's not pollute the source tree with __pycache__ files.

src/test/meson.build

@@ -15,6 +15,7 @@ test_env.set('SYSTEMD_LANGUAGE_FALLBACK_MAP', language_fallback_map)
 test_env.set('PATH', meson.project_build_root() + ':' + path)
 test_env.set('PROJECT_BUILD_ROOT', meson.project_build_root())
 test_env.set('SYSTEMD_SLOW_TESTS', want_slow_tests ? '1' : '0')
+test_env.set('PYTHONDONTWRITEBYTECODE', '1')
 
 if efi_addon != ''
         test_env.set('EFI_ADDON', efi_addon)