fix(qa-tests): Bring back decoding of exception field

Partial revert of fab085c15f

subprocess.run(..., text=True) is not sufficient to make the TimeoutExpired exception stop using byte buffers. This is a known issue, see: https://github.com/python/cpython/issues/87597

test/functional/feature_framework_startup_failures.py

@@ -49,7 +49,7 @@ class FeatureFrameworkStartupFailures(BitcoinTestFramework):
             print("Unexpected child process timeout!\n"
                   "WARNING: Timeouts like this halt execution of TestNode logic, "
                   "meaning dangling bitcoind processes are to be expected.\n"
-                  f"<CHILD OUTPUT BEGIN>\n{e.output}\n<CHILD OUTPUT END>",
+                  f"<CHILD OUTPUT BEGIN>\n{e.output.decode("utf-8")}\n<CHILD OUTPUT END>",
                   file=sys.stderr)
             raise