ensure function to test really exists
ensure function to test really exists

in case of a typo or mistake done by the test writer, non-existing
functions might generate false positives

example:

```
function check_input()
{
  if [ "$1" == "bad" ]; then
    return 1
  fi

  return 0
}

athena.test.assert_return.expects_fail check_ipnut bad
```

this was expecting the method to fail on input 'bad' but because of the
typo, any change on the `check_input` wasn't tested and so someone might
change the `check_input` behavior without getting a failed test

upside: this might save developers who accidentaly mistype a function or
        forgets to refactor a test if a function was renamed

downside: this increases the runtime for assertions a bit

athena-oss/athena