Implement nvim_command_output with `execute({cmd},"silent")`.
Behavior changes:
- does not provoke any hit-enter prompt
- no longer prepends a newline char
- does not capture some noise (like the "[New File]" message, see the
change to tabnewentered_spec.lua)
Technically ("bug-for-bug") this a breaking change. But the previous
behavior of nvim_command_output meant that it probably wasn't used for
anything outside of tests.
Also remove the undocumented `v:command_output` variable which was
a hack introduced only for the purposes of nvim_command_output.
closes#7726
It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.
