Support parallel_tests with multiple commands

[BenoitMC]

Currently, when using parallel_tests with 2 different test frameworks (eg RSpec + Cucumber), the guess is the same for the both commands and the second command results overwrite the first command results.

This PR change the guess to something like RSpec (1/2) instead of just (1/2) to have different command names and merge the results correctly.

[pboling]

I hope this can be merged!!