not all subtests are run when running a subset of tests

[amunger]

Testing #21495

from the repo in the linked issue

1. run the full suite, notice that all test_even tests are run
2. change the range in test_subtests.py
3. Use the testing sidebar to run tests within NumbersTest
   🐛 only test failure shows

[eleanorjboyd]

Yes, this is an ongoing behavior. We need to decide what will be the desired one since subtests are only known on run and not discover.