limit_memory for parametrized tests

Handle parametrized tests requiring differing memory limits. Currently, these tests need to be duplicated but we could use a fixture to apply dynamic memory limit or allow limit memory to be a list of limits.

@pablogsal ideas?

```python
@pytest.mark.parametrize("count", [1, 1000])
@pytest.mark.limit_memory("1 MB") # limit differs per parameter
def test_a(count):
assert 'a' * count
```

Start by reading the pytest limit_memory marker and how it interacts with parametrized tests. Compare the fixture-based and list-of-limits approaches, then add coverage for the parametrized example; done means differing parameters can use differing memory limits without duplicating the test.

Written by the indexing model from the issue text.