oldtests: Run nvim using --headless to avoid requesting user input

Certain functions (e.g. feedkeys(…, 'x!') and input()) will attempt to
read from stdin, which shouldn't be expected to work during oldtests.
In Debian/Ubuntu's build environment, it explicitly can't work because
/dev/null is redirected to stdin, which causes read_error_exit() to
exit.

Running oldtests with --headless prevents nvim from setting up its input
handling, thus avoiding the problem altogether.

Reference #6794

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