[REBASED !32] Fix quoting issues in t-xdg-open.sh

This fixes autotests with Dash prior 0.5.11 and Bash of any version.

See also: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=975228

Original patch by Nicolas Guriev.

Rebased and updated version of !32

Note: while I was at it I also decoupled local definitions from variable assignment

Closes: !32