Add automated tests for sheet command

[iamazeem]

sheet command has been introduced with 9c5f13d56b798359ab4eb6f1af02d97d76071a96.
Need to add automated tests for this.

[iamazeem]

@liquidaty: Here are some observations during testing:

Truncated column and values

Small screen crash/segfault (corrupted size vs. prev_size)

Header row issue with filter command

[iamazeem]

Just verified that the filter command output is just not a visual glitch.
The header in the captured output is not correct.

Steps to reproduce:

$ cd zsv/app/test
$ make test-sheet
$ tmux new-session -x 1000 -d -s test-sheet "../../build/Linux/rel/gcc/bin/zsv_sheet worldcitiespop_mil.csv" 
$ tmux set-window-option -t test-sheet prefix none
$ tmux send-keys -t test-sheet "f"
$ tmux send-keys -t test-sheet "sarmaj"
$ tmux send-keys -t test-sheet Enter
$ tmux capture-pane -t test-sheet -p > test.log
$ cat test.log
1         ir        sarmaj-e  Sarmaj-e  13                  34.3578
2         ad        aixirival Aixirival 06                  42.466666





















1
$ tmux send-keys -t test-sheet "q"