etc/schroot/debci/fstab: improve schroot isolation

* etc/schroot/debci/fstab: improve schroot isolation
  - do not share /tmp with host
  - mount tmpfs at /dev/shm as quite some software needs it
test_shell_best_practices.sh: skip when checkbashisms is not available.

* test_shell_best_practices.sh: skip when checkbashisms is not available.
  - You don't want to install devscripts and all its dependencies when
    running as-installed tests
  - checking for best practices is only useful in development environments.
Use unified diff in "Change in the base system since last run"