Request support for multiple local run tests on shared environments.

[squash-merge]

When a test is already running with -DSELION_SELENIUM_RUN_LOCALLY=true, trying to run another test on the same machine fails to start with error that the hub port 4444 is already in use.

This happens in shared environments, and in environments where there is already a grid setup but a need arises to test with locally spawned grid. (See #129).