tests: Add shell tests

This adds the following tests:

help tests:
shell_exec(help): 0
shell_exec(help dummy): 0
shell_exec(help invalid): -22

select tests:
shell_exec(select): 0
shell_exec(select dummy): 0
shell_exec(select invalid): -22

module tests:
shell_exec(dummy cmd1): 0
shell_exec(dummy cmd1 arg1): -22
shell_exec(dummy cmd2 arg1): 0
shell_exec(dummy cmd2 arg1 arg2): -22
shell_exec(dummy cmd3 arg1 arg2): 0
shell_exec(dummy cmd3 arg1 arg2 arg3): -22
shell_exec(dummy cmd4 arg1 arg2 arg3): -22
shell_exec(cmd1): 0
shell_exec(cmd1 arg1): -22
shell_exec(cmd2 arg1): 0
shell_exec(cmd2 arg1 arg2): -22
shell_exec(cmd3 arg1 arg2): 0
shell_exec(cmd3 arg1 arg2 arg3): -22
shell_exec(cmd4 arg1 arg2 arg3): -22

app handler tests:
shell_exec(cmd4 arg1 arg2 arg3): 0

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>