Add test that :!! repeats last command

This was fixed some time ago.  This is also what broke :!.
Don't crash :!! when no previous cmd available
Fix :! command

It invoked :!! after :!! was fixed.
Remove empty teardown from commands/input_tests
Make one of engine/cmds assert more informative
Merge branch 'fix-single-emark-command'

* Fix :! command.
* Add tests for :! and :!! commands.