Make t/156-fullscreen-focus.t easier to work with
Make t/156-fullscreen-focus.t easier to work with

- Add routine that will refocus the expected window on test failure.
Thus, failure on one test will not make others fail.
- Remove some redundant commands, prefer fresh_workspace for screen
changing.
- Kill previous windows between sections if the next section does not
depend on the previous layout.

i3/i3,Airblader/i3,Airblader/i3,stapelberg/i3,Airblader/i3-original,stapelberg/i3,i3/i3,stapelberg/i3,Airblader/i3-original,Airblader/i3-original,Airblader/i3,i3/i3,Airblader/i3,Airblader/i3-original,stapelberg/i3,Airblader/i3-original,i3/i3