Add process option

[mnocon]

This PR makes it possible to specify the number of processes that will be run in parallel - we need this to limit the number of processes to 1 as a workaround (until we find a fix for the session issue).

Example usage:

~/Desktop/Sites/v4 on master !689 ?191 ❯ vendor/bin/ibexabehat --process=1 --profile=browser --suite=admin-ui --tags=@test --dry-run                                    took 9s  14.17.0  3.0.0 at 10:27:24
- 2 test classes into the queue.
- Will be consumed by 1 parallel Processes.

~/Desktop/Sites/v4 on master !689 ?191 ❯ vendor/bin/ibexabehat --profile=browser --suite=admin-ui --tags=@test --dry-run                                                  ✘ INT  14.17.0  3.0.0 at 10:27:12
- 2 test classes into the queue.
- Will be consumed by 6 parallel Processes.

~/Desktop/Sites/v4 on master !689 ?191 ❯ vendor/bin/ibexabehat --process=2 --profile=browser --suite=admin-ui --tags=@test                                          took 1m 50s  14.17.0  3.0.0 at 10:05:10
- 2 test classes into the queue.
- Will be consumed by 2 parallel Processes.

[sonarqubecloud]

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[mnocon]

Replaced by ezsystems/BehatBundle#296