tests/desktop-app-info: Make sure scripts writes to the right end of the pipe

pipes need to be written in the write end, not the read end.

This commit makes that change in a recent test added.

Might help with this error message I'm seeing in CI:

/tmp/bin-path-H1UQT1/gnome-terminal: cannot create /proc/38961/fd/6: No such file or directory

Helps: #2781 (closed)