allow command logging in mocked CAF::Process

To log all mocked commands that are called via CAF::Process, do
```
use Test::Quattor;
$Test::Quattor::log_cmd=1;
```
To log all mocked commands that are called via CAF::Process (t)output, do
```
use Test::Quattor;
$Test::Quattor::log_cmd_missing=1;
```

This also fixes a bug that the mocked (t)output should return at least an empty string.