added new output to diaplay command to rerun all failed tests

Added a new output section to the base formatter that will output a single command to rerun all failed tests.

We have a huge testsuite that runs for a long time period.
When something brakes our code we'd like to be able to rerun all failed tests with a single command without effort until all our tests are running, without having to rerun the whole testsuite.
Therefor we will need to rerun the whole testsuite only after fixing all problems, to check for regressions.

This small patch adds that functionality. Copy&Pasting the last output line enables one to rerun all failing tests with a few clicks, without the overhead of passing tests.