test: use pytest with xdist where available

meson.build

@@ -292,6 +292,12 @@ if get_option('tests').enabled()
 		env.set('LD_LIBRARY_PATH', fs.parent(lib_libwacom.full_path()))
 		pymod.find_installation(modules: ['libevdev', 'pyudev', 'pytest'])
 		pytest = find_program('pytest-3', 'pytest')
+		pytest_args = ['--verbose', '--log-level=DEBUG']
+		# pytest xdist is nice because it significantly speeds up our
+		# test process, but it's not required
+		if pymod.find_installation('python3', modules: ['xdist'], required: false).found()
+			pytest_args += ['-n', 'auto']
+		endif
 		pytest_files = [
 			'test_data_files.py',
 			'test_libwacom.py',
@@ -301,7 +307,7 @@ if get_option('tests').enabled()
 		foreach f: pytest_files
 			test('pytest @0@'.format(f),
 			     pytest,
-			     args: ['--verbose', '--log-level=DEBUG', meson.current_source_dir() / 'test' / f],
+			     args: pytest_args + [meson.current_source_dir() / 'test' / f],
 			     env: env,
 			     suite: ['all'])
 		endforeach