tests: Be more careful with stdio
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tests: Redirect g_debug() to stderr
This avoids it corrupting TAP machine-readable stdout in older versions of GLib.
Putting _srt_tests_global_debug_log_to_stderr() in its own translation unit avoids dependency issues when linking statically, particularly in the scout environment.
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tests: Consistently run s-r-launch-client with /dev/null on stdin
This avoids it altering the settings of a controlling terminal, if any.