twister: Add choice for tests sorting into subsets

[PerMac]

Currently, subsets for twister testing are created based on a dictionary sorted by platform name. This is inefficient for on-target tests in setups, where multiple various platforms are available and on-target tests can be executed in parallel on multiple boards.
I believe the execution time for on-target runs will drop down when the subsets are based on test name sorting.

The way how sorting is done now can stay as a default. However, it will be good to be able to choose test-name-based sorting. I think being able to choose a sorting variant will be beneficial for on-target tests.

[PerMac]

@galak @nashif I can add a simple patch that will change the test order if --device-testing is set. I tested it locally and I had a significant difference in total execution time in my hw plan. The execution went down from 2h to 1h. This can be really helpful for executing on-target test when having multiple various platforms connected. E.g I have 4 different types of boards connected, 2 pieces of each. Now the benefit of parallel running on various platforms cannot be used and we are wasting processing power. Maybe this patch can go into 2.5 as bugfix? Edit: This relates to running in subsets