Set PYTHONIOENCODING when running tests

For instance, on Windows, if the terminal is in cp65001, the subprocess
output is not encoded correctly and it results in error when running
unit test.

unittests/allplatformstests.py

@@ -471,7 +471,7 @@ class AllPlatformTests(BasePlatformTests):
         # Ensure command output and JSON / text logs are not mangled.
         raw_output_sample = '\x00\x01\x02\x03\x04\x05\x06\x07\x08\x0b'
         assert raw_output_sample in tests_command_output
-        text_log = Path(self.logdir, 'testlog.txt').read_text()
+        text_log = Path(self.logdir, 'testlog.txt').read_text(encoding='utf-8')
         assert raw_output_sample in text_log
         json_log = json.loads(Path(self.logdir, 'testlog.json').read_bytes())
         assert raw_output_sample in json_log['stdout']

unittests/baseplatformtests.py

@@ -85,6 +85,7 @@ class BasePlatformTests(TestCase):
             # XCode backend is untested with unit tests, help welcome!
             self.no_rebuild_stdout = [f'UNKNOWN BACKEND {self.backend.name!r}']
         os.environ['COLUMNS'] = '80'
+        os.environ['PYTHONIOENCODING'] = 'utf8'
 
         self.builddirs = []
         self.new_builddir()