test: Do not write Python bytecode to source directory

This change prevents writing `share/rpcauth/__pycache__/*.pyc`, which is
especially useful for out-of-source builds when the source directory is
supposed to be read-only.

test/util/rpcauth-test.py

@@ -21,6 +21,7 @@ def setUp(self):
         with open(config_path, encoding="utf8") as config_file:
             config.read_file(config_file)
         sys.path.insert(0, os.path.dirname(config['environment']['RPCAUTH']))
+        sys.dont_write_bytecode = True
         self.rpcauth = importlib.import_module('rpcauth')
 
     def test_generate_salt(self):