unittests: use utf-8 encoding for child processes

Ensure that unit tests will be able to parse UTF-8 output of "meson test".

run_unittests.py

@@ -1600,6 +1600,7 @@ class BasePlatformTests(unittest.TestCase):
 
         p = subprocess.run(command, stdout=subprocess.PIPE,
                            stderr=subprocess.STDOUT, env=env,
+                           encoding='utf-8',
                            universal_newlines=True, cwd=workdir, timeout=60 * 5)
         print(p.stdout)
         if p.returncode != 0: