Add Support for a custom failure message for test statements

[dahlj]

It would be sweet if we could define an optional error message that is included in the test result for failed test statements. If given, this would replace the generic "expected / actual"-message in the result file.

Suggestion:
call enterValue "3"
test displayedValue = "3.00" [$actual$ is not a valid output, $expected$ should be displayed because all number shall have two decimal places in this screen]