tests: shell: Review test-cases for destroy command

Having separate files for successful destroy of existing and
non-existing objects is a bit too much, just combine them into one.

While being at it:

* No bashisms, using /bin/sh is fine
* Append '-e' to shebang itself instead of calling 'set'
* Use 'nft -a -e' instead of assuming the created rule's handle value
* Shellcheck warned about curly braces, quote them

Signed-off-by: Phil Sutter <phil@nwl.cc>

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