shell-extensions: Don't run tests if no user shell extensions installed

The tests check the user extensions in the running shell. This won't work if
there aren't any installed.

Fixes #728
1 job for shell-extensions-tests in 18 minutes and 13 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #354998
build-gnome-software

00:18:13