gh-109276: libregrtest: use separated file for JSON (#109277)

libregrtest now uses a separated file descriptor to write test result
as JSON.  Previously, if a test wrote debug messages late around the
JSON, the main test process failed to parse JSON.

Rename TestResult.write_json() to TestResult.write_json_into().

worker_process() no longer writes an empty line at the end. There is
no need to separate test process output from the JSON output anymore,
since JSON is now written into a separated file descriptor.

create_worker_process() now always spawn the process with
close_fds=True.

Lib/test/libregrtest/single.py

@@ -271,5 +271,9 @@ def run_single_test(test_name: TestName, runtests: RunTests) -> TestResult:
             print(f"test {test_name} crashed -- {msg}",
                   file=sys.stderr, flush=True)
         result.state = State.UNCAUGHT_EXC
+
+    sys.stdout.flush()
+    sys.stderr.flush()
+
     result.duration = time.perf_counter() - start_time
     return result