Add support for test filters

[dfabulich]

This PR deletes some comments saying that what I'm doing here doesn't work. It just worked on my machine, so I'm not sure what's up with that!

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Cursor did this. I tested it with a local skipstone build, testing skip-foundation with skip test --filter URLTests

[✓] Build Project (12.6s)
[✓] Test project (31.58s)
| Test     | Case     | Swift | Kotlin |
| -------- | -------- | ----- | ------ |
| URLTests | testURLs | PASS  | PASS   |
|          |          | 100%  | 100%   |

[✓] Tests 1 / 1 passed
[✓] Skip 1.8.8 test succeeded in 44.21s

[marcprux]

This will be fabulous if it works! Not being able to filter on tests is very tedious for the packages that have many test cases (skip-foundation and skip-ui, notably).