test_x11_remote: Print xterm log to investigate intermittent failure
test_that_stbt_run_exits_on_ctrl_c: Fix intermittent failure

I saw this test fail on Travis, possibly because we added parallelism to
the Travis test-runner in b46b3939 and now 5 seconds isn't enough for
the killing process to be scheduled when the Travis host is under heavy
load?
make check: Silence parallel's citation notice

Otherwise you see this in the `make check` output:

> When using programs that use GNU Parallel to process data for
> publication please cite:
>
> O. Tange (2011): GNU Parallel - The Command-Line Power Tool,
>
> ;login: The USENIX Magazine, February 2011:42-47.
>
> This helps funding further development; and it won't cost you a cent.
>
> To silence this citation notice run 'parallel --bibtex' once or use
> '--no-notice'.