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.