Improve location of test files for `test` command

[hvanz]

From https://github.com/informalsystems/atomkraft/pull/199#discussion_r1032401187 :

We always create a single test_* file in tests directory for each test trace ... call. So for a directory passed as a trace, it will create a tests/example_<TS>/test_example.py - a directory containing a single test.

Why not:

• put single test_*.py files always in tests/ directory?
• Or, we replace tests/example_<TS>/test_example.py with tests/example/test_example_<TS>.py. But then we should do the same for individual traces?

I [Rano] like (1) if we don't care about tests/test_*py. I like (2) because it organizes the tests directory.